Mastodon

I think Matrix or another (self-) hostable mesenger (or even a Signal group) would be better for this since it can and should be encrypted. I don't really see a use for something public when you want it to be family/friends only. And with Masto it's very probable that everything your family posts will be gobbled up be secret services, AI companies, Google/Meta etc. Encrypted Matrix would be private and afaik aware, there are no features Masto has that Matrix doesn't (can post images, text, video, reactions etc).

I did this, we used https://masto.host/ and prevented other registrations. You can also make the posts invisible to people who aren't logged in

I too am thinking about it, but I fear my family is too far gone with Bellshill accounts on Facebook and tik tok...

You may only want to communicate with your family, but does the rest of your family only want that too? Otherwise you’re trying to convince them to now manage two separate services to keep in touch with you and everyone else.

Would it be better served by a chat group on signal or something else instead?

Movim https://movim.eu/ could serve a dual purpose for you if you don’t yet have chat which often make sense for certain non-tech-savvy circles. It can be an all-in-one application for chat + posts (short form, long, with media, & now whatever those short videos are called)—& this better mimics how many interact with Facebook. It isn’t ActivityPub, but it is FOSS / decentralized (can join or disable connection to other pods/servers). There aren’t chat applications integrated together with ActivityPub—& Mastodon doesn’t provide E2EE.