Mother-in-law troubles.

I'm getting married on April 18, 2020 in my parent's backyard on an almond ranch in Central California. My fiance is five years older than me & is from a nearby town. A little back story: He has a younger brother & his parents struggled financially when they were young. His dad was an alcoholic & his mom stood up to the task of playing the mother & father roles in their kid's lives. His dad is 15 years sober, but acts like he has the worst life imaginable. He complains about everything & is so mean to my fiance & myself. My fiance stands up for us as much as possible, but the man just doesn't stop. He's constantly putting my fiance down & just being negative. My fiance's mom is super controlling & rude to me. I don't think she likes me because she feels threatened maybe. She demands to see us every other weekend since we're living with my parents until we get on out feet to move out. She made the comment along the lines of, "feeling like a divorced parents trying to gain custody of my own child," at dinner last weekend. She's always making comments like that or about my or my fiance's weight. She lost 30 pounds on Weight Watchers & now acts like we are disgusting since we aren't exactly a healthy weight. She told my fiance that he needed to get checked for "diabetes because he's so fat." It is so hard for me to not go off on her, but I don't want to ruin the chance of eventually having a relationship with her. She wants to help with the wedding, but I can't stand her snarky, mean comments. I don't know how to handle this woman. My fiance knows that she's a little off the rails, but it's his family, so he has a hard time putting his foot down. He was very sheltered until he met me, so he's still trying to learn how to have confidence in himself since his father obviously has crushed any hopes of him having a high self esteem as a child, or even an adult, for that matter. So I guess I just need some advice on how to approach my future in-laws without completely ruining any hopes of a possible relationship with them. I feel like this needs to be a conversation before we get married or have children because as of right now, I am 100% not comfortable with leaving my future children at his parent's house to babysit or even just visit.