I dont feel like me and my fiance are going to be together in the long run last time I was at his…

I dont feel like me and my fiance are going to be together in the long run last time I was at his sisters house she didnt know a bottle was pose to be warm and she knew sarah used the bathroom but didn't change her diaper and she didnt feed her all the way and she sat there and cried her eyes out and then left her on the edge of the bed but im the bad person because I said she cant go over there by herself im not understanding he literally treats his sister like his wife