I share a birthday with my SIL. I had been in the family for YEARS at this point and had two children, our birthday twinning was no secret. My husband one day said we were going to go celebrate our birthdays at MILs place, which was totally fine with me, at this point I had no issues any with these people. SIL and her family live several hours away and I was happy to share the day. Upon arrival I knew exactly where “our” celebration was going to pan out. Hubby had been invited to celebrate SILs birthday, not mine. He just assumed that it would be for both of us. I didn’t say so much as one word when I saw the big cake in the middle of the table saying HBD SIL (in full form, with her name), no mention of me. I sat through an enthusiastic round of the birthday song, and watched her get her gifts and cards. I had enough quickly after that, not even a causal happy birthday greeting from anyone in his family. I gathered up my kids and as we were leaving MIL hit me with “oh I guess it’s your birthday too” and walked away. This was the beginning of the end of my relationship with most his family. I could tell stories for hours about what finally nailed the coffin shut, mostly on my relationship with SIL, but that pretty much ended me being welcome for any occasion. I know it kind of seems petty, but it was a snub that hurt my feelings none the less