Soon to be married couple with children and family...?

This is the situation...I have a 5 year old daughter, he has an 8 year old daughter. His mother lives with him and when I move in, I want my youngest sister to live with me. I have taken care of her for the last 2 years. He has asked me to move in and marry him, even though I already live with him half of the time. I sometimes want to but then think that the other family members will cause problems between us. I have a tiny chihuahua, he has a big pitbull, they get along fine. I can't get my doggy to get potty trained though. This morning his mother was very upset because the doggy pooped in the restroom floor and "someone" I have no idea who, cleaned it up with a towel. She told him and he told me. Should I just get rid of the doggy? If we had no family memers living with us, there would be no other opinions only mine and his. I don't know what to do? Its a huge step and I don't know if Im ready for it or not. I don't want to get into something and later regret it.....please help me!!!!!