YES, it is weird! Guys, you are feeling uncomfortable because there is something to feel uncomfortable about. After a while of this... An otherwise great guy with one very strange thing... You start to feel like YOU are the crazy person. It wears you out, wears you down, digs at you, and it's like you cant get past it. Right?
There is a really good reason for this. What he is doing is wrong. Staring at his sisters boobs is gross, yes. Yes, him resting his hand on her thigh for 20 minutes is strange. Yes, him having his sister sit with him at the hospital but making you wait outside is upsetting.
Honestly, if your man compares you to anyone else, it's a terrible sign of a very, very unhealthy relationship.
You need to know that in the end, you have 2 choices. You can live with him behaving like this, or you can separate from him. He is clearly not ready to let go of his sister and move into an adult world with an adult woman love... It is really, really sad, especially because you love him and think he is an otherwise good man.
Good luck. Don't forget that you need to take care of yourself. Do what's best for yourself, and don't waste time on indecision. You don't get to live forever.
