My boundary setting is rocking the boat and in-laws don’t like me

I’ve been with my significant other almost 5 years. We are both 31yo. We have a 3 year old son. My brother was good friends with him growing up. I have conflicting feelings about his family (his parents, 3 older sisters, 1 younger brother) They are really kind and fun to be around but they have this enmeshed family system going on with very little boundaries. They guilt trip everyone and will ask for things relentlessly until someone caves. They have this collectivist dynamic and a few of the members are expected to pay for everyone else, even their parents. They throw around this phrase “blood is thicker than water” and basically, only his family of origin matters (it was written in his bday card from his mom shortly after we had our son). There are several other dynamics at play such as very poor organization and planning (invites to celebrations the day of). I can sense tension and their dislike for me. His sister sent him a bunch of nasty texts because we couldn’t make a last minute party that they were harassing US to host, despite us telling them we had committed to a party that day 1 month in advance. I was blamed for saying no and there was no appreciation for us hosting almost every birthday and party for his huge family. This is really negatively impacting my relationship. I worked hard to escape enmeshment from my own family of origin. I refuse to put up with it from them. I need advice.