relationship in trouble ( a deeper understanding)

I have the ill-est feelings toward my boyfriend because before we got together he tried to have sex with my cousin. After we got together he admitted it to me but when I asked them about it before we got together they both lied and said they didn't which makes me believe they did actually have sex and he's just lying to me about it. Now I literally hate my cousin because she was the one person I told how deep my feelings for him were. I feel like if she was a real cousin then she would have told me because I could have decided if I wanted to go any further with him but she never told me and I hate him at times because of all the people in the world why would you try to have sex with my cousin. I know I can't get mad because we wern't together but I am.
To make a bad situation even worse they start calling each other brother and sister and that right there really pissed me off. I feel like they just want a reason to be in each other lives and would have sex again (if they did the first time) if they could get away with it.
Now they don't talk, well at least they say they don't but I don't trust it. I feel like he has some type of feelings for her and she most definitely has some feelings for him but they both keep denying it. I get mad when they are in the same room. When they look each other way my blood boils.
How do I fix this relationship before it's too late please help me I'm losing my mind!!!!!!!!!

How willing are all the concerned parties willing to work all the issues out? Are everyone ready to face challenges and work on them? What will help everyone involved works to the equal level of respect and trust? Would you all be willing to consider a mental health professional to assist in leading all of you to a mutually acceptable solution?

OK, I have no credentials to give professional mental health care, so please take what you like and leave the rest behind. What's been written is just my opinion, a consumer of the mental health system. Take care. Hope all of you involved come to a satisfactory, mutually acceptable solution.

well me and my boyfriend discuss it and me and her discuss it but not all three of us because they claim they don't talk to each other. i don't for one minute believe that so it's hard for me to attempt to talk to them together without wanting to hit one of them in the face. i know that's not normal and it isn't healthy for my relationship but that's how bad thing have gotten for me. but this issue has to be resolved and it scares me that the only resolution i see is me leaving him.

If the trust is breeched it is better to move out and move on. Like you said you are 100% sure they are lying then why to keep up with this rubbish. you deserve better girl

* breached

I keep telling myself I'm done but every time I try to leave I can't. I know I sound like the typical dumb girl that's in love but there's a lot more to him than the bad. I'm just venting about the bad because I barely have people I can talk to about what's going on in my head that won't judge me.

wow that's tuff

Yeah but I'm learning to slowly get over it. Slowly but I can't shake the feeling that they're lying. It's tough because I don't want to end a relationship on an assumption.
He's been honest about the situation this far. It's just the way I think.

could you provide a timeline of events (that did happen and that you think might have happened)?

Well this all transpired in 2009 around late November. When I originally asked both of them did they sleep together, a day or two after I believe it happened they both said no and they were being secretive about something. About 4 days later they both removed me on facebook and would be talking about it in code so no one would find out what they were really talking about and after that I let it go because after all he wasn't my boyfriend at the time. It wasn't brought back up until 2011 and that's when he told me that they were about to have sex but they didn't. I was fuming and I hated him and her for a long time because I felt like they never had to lie to me from the beginning. He lied because he knew I wouldn't be bothered with him anymore and she lied because she thought she would lose me as a cousin (these are their words when they discussed it with me in January of 2012)

Every since then they haven't been around eachother nor do they talk to eachother but like I said I think that's only when I'm around them. I love both of them but I don't see me and him having a healthy relationship because I don't believe that "nothing" happened. I believe that's all he was willing to tell me because of my reaction when he told me they were about to have sex but didn't.

Our relationship is in so much trouble that he's suggested counseling and paying for it but I don't care to go anymore. I just want to be free and take care of my children. The only reason I'm even still carrying out this relationship is for the kids and he knows that I love him but I know one day soon I'll be gone and it'll be all his fault and if he was telling the truth(which I doubt) then it'll be my lost I guess. NOT!!!!

Shouldn't have lied in the first place and we wouldn't be here right now.

So you are upset with them because they may have hooked up before you and bf started dating and lied about it.
Is that correct?

Getting upset over the dishonesty that was discovered in 2011 understandable. Although 2 years have passed and enough time to try and re-establish trust.

Anger toward cousin because she knew you were crushing on him in 2009, also understandable. Although again, have any attempts been made at rebuilding trust between you and cousin since 2011?

What I don't understand is why either of those things are causing this much problems in 2013...
3 years have passed since they "supposedly" slept together, and even then, anger and distrust should be directed at cousin, she betrayed you, not the future bf. I mean why NOT try and sleep with cousin? Is she hideous?
He was single afterall and didnt betray anyone, his sin is lying about what transpired.

I think that if you are not going to accept his offer to go to couples counselling that you should break it off.

Years have passed and you have not resolved this on your own, refusing help to do so will only lead to more years of the same type of misery and distrust for both of you.

2009 is a while ago. Is that the only thing, hindering moving forward towards a loving, trusting relationship? Does he behave secretively, now? Are you worried, he'll run into the arms of your cousin, instead of you? Is you cousin, currently actively involved with someone else? Has she been dating, since this 2009 incident? Is your bf, her only love interest? Is he, her only friend?

Sometimes, fear based logic, can push those we love away. It's great that you've had this discussion with him, about this incident and you've voiced your displeasure with the whole thing, but having it recur, even in your mind, over and over, doesn't bring a relationship to the next stage/level. It's not going to help your relationship grow.

It's a repeating, negative tape, rolling in your mind. Therapy, CBT, can help that.

Thanks everyone for your input. I really don't know what to say anymore about the situation. I guess I'm just jealous and this whole situation has made me bitter but I'm dealing with it and I agreed to go to counseling. I'll be updating you and hopefully I can finally put this whole situation to rest.