In-laws

My relationship with my inlaws is currently driving me a bit mad and I'm looking for some advice! I try really hard to get a lot with them but so much has happened and I just feel angry.

So I live in NZ and my partners family are Maori. I am European and have had them constantly bring up how white my child is. I have even heard them talking to my 3 yr old son about his skin colour. They refuse to call him by his first name, instead they call him by his middle name. They told me when my son was a few weeks old that as his middle name is moari its better than the first name we chose and they will call him by that.

I was put under a lot of pressure to go and stay with them (they live over an hour away) when my son was only 6 weeks old. I really didn't want to go but really wanted a good relationship. One morning I was feeding my son and my FIL came out and asked to take my son for a little bit so I could get some sleep, I was unsure but let him. A little while later my MIL comes back with my baby laughing, saying she found the perfect way to shut him up and that she had let him suck on her nipple. I was SO upset and confused. She laughed at me and said she didn't even consider my feelings and did it to bond with him. I was so upset I couldn't face them for weeks.

I chose to put the incident aside and we stay with them once a month(ish). I never left them alone with my son while I was breast feeding again. They have looked after him for a few hours here and there when we have needed a sitter but they have no respect for how we parent our son. They keep him up until 10pm and always make out any rules we have are extreme. My father in law even deliberately shared a drink with my son once hoping to pass along a bug he had to "strengthen my sons immune system"

My MIL said to me in conversation about how the law here says she could just take my son and I would need to take her to court to get him back.

So my main issue now is my son is 4 and i am pregnant. They are putting a lot of pressure on us to let our son stay with them for a few days because "he needs to spend some time away from us" I have said no each time they have bought it up, but they continue to do so. They even talk to my son about how they need to come pick him up and take him away for a few days. It's really stressing me out. I hate the fact I feel like I am the bad guy for not wanting my son to stay with them unsupervised!!

I guess I just need some advise on how to handle this???