Nothing you have said suggests that there would be any romantic involvement between the two. I think you are allowing paranoia to get the best of you and the bias in your perspective is causing others to prematurely suggest that you break off your relationship and intended marriage. You have not provided enough information here in order to even speculate, let alone give you a definitive response.

I think that from what you are saying he is nothing but a momma's boy. Although it's possible that these could all be early signs of an incestuous relationship, it's far more likely that he just really appreciates his mother - a little too much maybe. Everything you have pointed out thus far (the showering more; taking care of himself; willingness to assist her) realistically is indicative of nothing but utter appreciation. If you were to drop the 'sleeping together' incident for a moment you would see that all of the supporting evidence fails to stand individually. He may just want to impress his mother and look on top of his game in order to prove to her what a great man she has raised.

That being said, there still could be some potential problems in your future judging by their relationship. He seems a little too concerned with making her happy and showing her appreciation. He is spending all his time and energy doing that and has accomplished this by spending less on you. Your the important one and you plan to marry him and you have to get him to acknowledge that if you do not want any future problems. Where is the husband/your father? If he is no longer with you that could be the potential cause of this problem. He may be trying to make up for him. The real problem however seems to be the mother if you ask me. While the fiance may just be naive and not attentive to this problem, the mother has lived long enough and experienced enough to recognize this type of situation. She most definitely has at least once in her life felt under cared for or unnoticed by a loved one and therefore I would say that this is purposeful. She may be acting in a passive aggressive manner and trying to fight over your fiance/her son's attention with you. She is obviously being selfish and the guy may not be aware. This holds true especially for 'mommas boys' as this is the result usually of a mother who cannot let her son go and therefore babies him his entire life. If her husband is out of the picture I'd day this is definitely what your seeing. In regards to the sleeping together thing, she is the one who told you about it not your fiance. What reason would she have to tell you that other than to make you worried and/or make you jealous. Most incestuous relationships are accompanied by guilt and shame and are hidden. This just doesn't seem like the logical conclusion to draw based on these facts. It is much much more likely that she just wanted to make you jealous and possibly uncomfortable and made a passive aggressive remark. Remember, this isn't your fiance telling you these things. You owe it to him to talk first before making a rash decision. There is a reason you must have wanted to marry him in the first place. I'm sure you can think back and notice some signs of him being willing to please his mother. What you won't remember is any other time you ever though there was an incestuous relationship occurring within your household. Please take the time to read what I said and give it some though