
Hi NeuroticallyNormal!
Welcome to PC. You have an interesting situation, but not one that is abnormal at all.
IMHO, it looks like your sister has taken on the role of "mother" in your life. Maybe at the time your mom passed, she took over that role because she loves you and wants what she thinks is best for you. Of course, as you have grown older, it's hard for her to let go of that role she has taken on.
You are kind of stuck in the middle here between sis and gf. I can see you are doing your best to keep the peace between them, and I know that's not an easy position to be in. I think it would be wonderful for you to express to each of them how much you love them and appreciate them both. Gently letting sis know that even though she has been a great sis and you love her for everything she has done for you, that you are now and adult and need to make your own decisions. Even if those decisions are not what she feels are in your best interest, you need to make them and succeed or fail, whatever it's going to be. That's how we learn and grow. Hopefully now that she is about to have her own child, she will be busy with the little one and not have as much time to think and worry about you and your life.
If along your journey, you have made some mistakes in your life, I can understand her being worried about you. If you have talked to her about the split with your gf and spoke in derogatory ways about the split, maybe she picked up on that and is worried you are making the same mistake again.
Of course the best time to speak to either of them about this is when they are in a good mood and you start it off with a positive statement and not accusing them of anything. Maybe, if you are more comfortable with writing, you can write to them instead of speaking face to face.
I wish you well with this age old problem. I hope your family can find a way to come together and accept each other.
Take good care!
Hugssss
J