You might be overthinking, but the run up to Christmas is a fraught time emotionally for lots of people, and that can overspill into all sorts of areas of life.
Personally, I'd probably find it easier to tell a few friends than to tell family, as it's less all-or-nothing. And then maybe see how that feels?
I also agree that 'here is someone I'm seeing' is easier than 'big revelation: I'm bisexual'.
However, as always on these threads, I do think people are too quick to say stuff like Generally speaking in 2018 people are fairly okay with it. It's not true. My family were not wonderful, and I was gutted when a friend (otherwise supportive and not at all homophobic) responded by telling me if I were really serious about things, I should stop trying to date women and find a nice man to have children with. You never know what people will say or do, and therefore you shouldn't let anyone pressure you into coming out when you're not comfortable - they don't have to risk the responses, and you do.
FWIW, despite coming out in my teens, I still had to do it all over again to my family in my early 30s, because they simply didn't listen/believe it.
I can relate to the 'eek, maybe I'm not' worries, too. I think it's very normal.