help with husbands family

okay guys i need some advice. my husbands family is just...unreal.

basically on sunday his little sister (5, who i’ve been around since she was 4 months old), his step mother, and my daughter (3 months) and i stayed at the house together and it just was a disaster. his little sister DOES NOT listen for ANYTHING to anyone. she’s the only girl out of 5 children, and the youngest. she says and does whatever she wants. his step mother doesn’t parent, and has quite literally, word for word said,”when there’s other people around or i’m in public, i’m not a mom my kids (the 9 & 5 year old) are everyone else’s issue. not mine.” just to give you some backstory.

so i told his step mother that i didn’t want Ava (his little sister) holding or feeding the baby, since she didn’t listen, and i told the 5 year old the same thing. i gave my daughter to his step mother and went to the bathroom, and i’m not going to lie, i took my time. probably was in there 3-5 minutes, and came back out and walked up to the couch, and there’s Ava, holding the baby, SHOVING FRUIT SNACKS INTO HER MOUTH, i screamed and grabbed the baby and pulled about 3-4 fruit snacks out of my daughters mouth. i snapped at his step mother, who was COMPLETELY clueless to what was going on since she wasn’t watching and was on her phone, and then LAUGHED when i explained what happened and just shook her head and said “kids”. fast forward an hour or 2, and i allowed Ava to hold the baby after i changed her, with the understanding that his step mother would be watching, and ran upstairs to pack our things. came back down and ava has the bottle and is feeding the baby and the baby is freaking CHOKING and his step mom is literally nowhere to be found. like she went to her room and left the 5 year old with my child, FEEDING her (she then posted pictures she took on facebook and said “being an auntie is so fun!!”). she’s constantly in the babies face, even after i tell her to stop, like she just doesn’t listen and her parents allow it to happen. anyway, my husband and i talked about it since we have to see them again saturday and i tried to tell him that basically i didn’t want his little sister or step mother holding the baby, feeding the baby, and i don’t want his sister all up in the babies face since she doesn’t respect me as a parent and her daughter doesn’t listen to a word i say. he FLIPPED and now we’re in this huge argument because he thinks that i’m being ridiculous and stupid.

i don’t know what to do like the thought of being around his family is at the point that i have full blown anxiety attacks and am a complete ball of nerves. like does anyone have any advice or am i being ridiculous or?

-a little background m 20, which is why they don’t respect me. and when it comes to his little sister it’s just like i don’t want the 5 year old kissing or honestly touching the baby since her mother literally doesn’t bathe her. she tells her to go shower and trusts she does it. her hair on sunday was so greasy it looked wet. hasn’t been brushed in god knows how long. she’s just FILTHY and now that she’s in school and around other kids i’m scared of her giving my daughter, who isn’t fully vaccinated some sort of sickness or bacteria that her immune system can’t handle yet.