It's fair that family is prioritized for important things. But, if it's trivial and he's missing your birthday for that trivial thing then that's not ok. Ask him why he chose that trivial action over being with you.

That's how it is in my marriage. If I had questions about it I would ask my partner and see how they felt. I'm not sure I understand your question.

In my life family takes priority. When you marry someone they are family. At that point they are as equal as a sibling. Priority within the family is a case by case basis on the situation.

Try not to let insecurities get in the way of your relationships. Trust that they're being faithful until they show you otherwise. Stick with it.

I personally wouldn't see them. If you're moving away this will only make it harder. Remember the good times and maybe reach out when things are calmer in your life.

I think the first step is to spend more time with her doing anything. When she's opened up more to you then bring up the subject privately and let them know how it hurt you. I'm sure your niece is upset and embarrassed as well. She's young, and probably needs to sort herself a bit on this issue. Keep being there for them as best you can.