Agree, liz, with Sparkle. Somehow you have gotten into a role that you are not liking. How can you dial it back, do you think?
I know it's not easy. My bestie extricated herself from a 7 year relationship not so long ago. There were two adult daughters who just weren't nice. They resented my bestie's presence, were very entitled, (he was rich) and gave her the hardest time for a lng time.
Interestingly enough, she and they eventually worked things out. She refused to engage in their drama after a while, and learned how to keep them from running the show when they were there, and that helped. When they saw that they couldn't bully her, things got better. Took a couple of years, though.
She left her SO for reasons having nothing to do with his daughters.
Would counselling help? Talking to someone with experience in family situations like yours who could give practical advice and tools to help you shift your role from chief cook and bottle washer to one with more equality?
Have you talked to your SO about it? Assume you have.
Just because it is his family doesn't mean that you can't change the dynamic to one with which you are more comfortable. He and you can be united in this.
Quiet, but firm boundaries, beginning with "we speak with respect to each other."
Anyway, my two cents. I feel for you. It's a tough one.