My Mom Doesn't Like My Autistic Step son

So I had recently made a post about preparing my step son for a new baby since we just found out I'm pregnant. He's 15 with Autism, he isn't very verbal and wear pull ups. My mom had always had a problem with me being with a man who has a kid and did not support me at all when I adopted him after we got married. She keeps saying he should be with his mom, who is a damn crack head. My mom also doesn't believe in Autism. She says my husband is just lazy and didn't want to teach his son to behave. I just can't believe her. She's super excited about this baby. We were staying over there because my mom didn't want to be alone during this chaos. My husband is a nurse and was at work and I decided to do some grocery shopping. I was hoping my mom could bond with my son while I was at the store. I was WRONG! She called me pissed off. Basically what happened was she decided to try to treat him like a regular 15 year old and made him take off his pull up. She said he's too old. He ended up having an accident on her couch and she yelled at him. I drove back there quicky and she was yelling at him and he was having a melt down. I told her not to yell at my son and she said I need to pay for cleaners to clean her house. First of all good luck finding cleaners during this time, and second she chose to take off his pull up. When my husband got back he yelled at her and said we were leaving. We left and my mom called me crying saying how can you just leave me here by myself. I'm your mother. Just a bunch of pity party B.S. we won't be back over there, and I personally feel like I shouldn't have to pay for the couch cleaning.