Don't want SIL to live with us

So, just let me start from the beginning.

My fiance and I lived with his mom in the in law quarters from June 2016-December 2018. We moved because we were expecting and they were FILTHY. They had bed bugs and kept getting lice because they didnt debug the furniture. Also his mom and 3 younger sisters would sneak into our part of the house and steal literally ALL of our food, steal MY clothes, and we had a huge jug in our storage closet with probably $75 of coins in it that was mysteriously missing when we were looking for it when we moved. When they would come in our space I was always paranoid we got their bugs and I would go on a 2 days deep clean. We only had to go into their part of the house to do laundry. Their part of the house was/is disgusting. They leave half eaten food out beyond the point of it molding, every dish is dirty and never cleaned and just sits all throughout the kitchen, they leave dirty laundry throughout the house (I found bloody underwear in the dining room multiple times), and their bedrooms all looked like a Hoarders episode. I grew up with chores and my mother insisted on having a clean house, it didnt have to be pristine but it needed to be CLEAN. No clothes thrown around, no food left out, if you spill something you clean it, and no clutter. Maybe that's why I'm overly disgusted by them.

Well we moved out into my parents house because well they offered and we didnt want to get stuck in a lease, and were planning on buying soon. We had our baby girl in August and now actively looking to buy.

Now to my conflict..I'm a stay at home mom because my fiance and I don't trust anyone except family to care for our baby. So he works, meaning all the money is his. He can do whatever he wants and I know that. We have been looking for homes and he keeps looking for homes big enough to also accommodate his oldest younger sister. But I 100% do not want to live in the same home as any of his family every again. She is 20 years old and doesn't work, she plays xbox and eats all day. I already have a baby to take care of, if she moves in I'll have to also parent her because she still acts like a 12 year old. I don't want to have to go behind her and clean her messes, and do her laundry and debug her because the house she lives in now is INFESTED.

What if we want to go on a vacation? Do we trust her to not demolish the house while we're gone or do we have to take her everywhere we go because she is incapable of caring for herself?

He says I'm being difficult and that I'm acting like I'm better than his family.

Is it so wrong to not want a filthy, bed bug ridden home? Is it so terrible that I would rather just have a nice clean home for my 3 month old? I want us to have our little family. I want to be a good mom for my daughter and a good wife for my fiance and take care of them. But I don't want to take in an adult "child".

I'm sorry if I come off as snooty. I just really dont know how to fix this issue..

**For the record..his mom wasnt doing us a favor. WE all rented that house and split the rent into occupants. So we paid for 2 people where as she paid for 5 and we also split utilities. They paid for the electricity, while we paid for internet, cable, and phone and then water. They wouldnt have gotten the house without MY good credit..sooo..who did who a favor...luckily and unluckily we had a landlord who didnt care about ANYTHING. He didnt care that the house was a disaster zone but he also didn't care what we wanted out of the lease early.***