I had the same kind of feelings a few years ago when attending my grandpa's 90th birthday party. I hadn't seen him or the rest of the extended family since my start on T, and it was kind of stressful for that reason. I guess I don't care too much about my extended family, but my grandpa and I are (well, were) born on the same day. As my birthday buddy I felt closer to him in a way. He didn't care one bit. We had a blast. Ate some food, and called it a day. Since then I've seen my extended family a few times with no stress about it at all. Usually they just tell me I'm lookin good or something. I say thank you, assume this is non-trans related, and move on.
I'm sure you're situation will go equally as well! They sound like a pretty open bunch.