Help with relationship!!

I have been in a relationship for 2 years and recently engaged. I can't help but feeling doubtful of my relationship, though. I have a 5 year old son from a prev. relationship and my fiance has a 10 yr old daughter from a prev. relationship. My 5 year old usually doesn't misbehave too often. He's an average little boy. My fiance's daughter has emotional problems (an attachment disorder, compulsive lying, and temper tantrums) My fiance's mother babysits our children when we go out to dinner or to run important errands. She dislikes my son. After running to the store on one occasion, we came home to his mother saying that she hated my son and that she thinks that he is stupid and socially retarded (these things were said infront of my son). If he spoke she would tell him to shut up. His mother looks through rose colored gles at her granddaughter (who consistently misbehaves) but treats my child like crap. She continued on telling me that she hates me and that the next time I leave town to go to work she would take all of my belongings and throw them away and I was never to come back. My fiance's mother handles our finances so he won't say anything to make his mother offended and it hurts my feelings that he didn't stick up for my son. He just agreed with her and said that "Yes, he does whine a lot. " and that he agreed "that he doesn't like when I have to go out of town to work." I feel as though they just want to control me. What should I do?