What do i do ?????

I need opinions from someone that doesnt know us. I'm 22 and my fiance is 24, we have a 2 year old baby together and are ttc baby 2. We started dating when i was 17 and we have been living with my mother since the beginning. My mother is my best friend and she and my fiance get along great and are good friends. But my mother had an accident some years ago and cant work so her income is from the state and it's not much.. my 19 year old brother who has autism lives with us too. This is only a 3 bedroom appartment so me, my fiance and son are all in one room. My mom wouldnt be able to afford food, rent and things if we left and she has often cried over how scared she is if we would move. My fiance wants us to move and be a family of our own in a few years but i dont have the heart to leave my mom. She helpes alot with our baby and kinda lives though me. We do everything together. We have been through this conversation over and over and i just want to find a way that makes everybody happy. What would you guys do ??