Very good question and welcome
There is two sides, one is that you dont live with your partners family you live with him meaning you are not obliged to mix with them. The other side is that if you persevere then people often do settle in with getting to know you and finally accept you.
The ideal situation you can manage (for the sake of your partner) is to balance your contact with them. Attend every second gathering then every third. When you attend sit with the most friendliest of all and dont move. You likely have tried to mix so that is now exhausted, its time for them to make an effort if they feel like it, if not then so be it, leave them as strangers.
In the following thread I highlighted the advantage of using wit.
In your case for example if one of these family members says to you "about time you attended a get together" You could reply "well if I was welcomed more I'd attend more". The desire in doing this is not to be sarcastic but to protect yourself by giving as good as you get. That is a major defence strategy that many people have the ability to do but if you dont and you are sensitive, you pay a price for your caring attitude. Being kind and sensitive in a family that dont accept you will mean they dont care and you are hurt. You might be a HSP Highly sensitive person, 20% of all humans are. That doesnt mean you are at at fault though. It means you have to develop strategies to survive better.
You only need to read the first post if you wish. Repost anytime in those threads or here. I'm here daily.