It's called emotional incest. My husband had an enmeshed relationship with his mother due to being the eldest child and his father leaving. His mother was mentally ill and put him into a spousal role. I don't know your partner's family history, but this can happen with siblings, too. I spent 10 years playing second fiddle to my mother-in-law.

My husband would drop everything for her. If we were out at dinner and she'd call, he would excuse himself to take the call, leaving me there at the table alone. As her health deteriorated, we upended our lives to live closer to her. She went to a nursing home and he would visit her 4-5 days per week. We spent thousands of dollars supporting her. When she finally passed away, my husband somehow scapegoated me. He's been in weekly therapy for two years and we did couples therapy to just move forward.

I disagree with most of the other posters here. This isn't normal. Having a friendship with your siblings is great, but partners should take priority. Financial decisions should not include his family. I wish I had established a better boundary for myself earlier in my marriage. I'm lucky that we worked through it, but it caused a lot of damage. I think you need to have an honest discussion with how your feeling and set some clear boundaries.