If you're all into games, fighting games are great targets for group cosplays with a diverse age range!
I'm 19, and I cosplayed as Fem. Vergil last year at Youmacon. My older sister went (in her twenties) as Trish. My "Tweenage" brother went as Albert Wesker, and my Dad (in his forties also) went as Ghost Rider. As it turns out, we all cosplayed as characters from Capcom's Ultimate Marvel vs. Capcom 3.
Games like Street Fighter, Tekken, Mortal Kombat
and other fighting games give you a large selection of characters to choose from, and usually have characters with various ages. So they're fantastic for group cosplays that involve multiple generations. HEhehehehehe.... also, ask your dad about Rampage.
You could go as Lizzy, George, and Ralph, since there's three of you!!
If you're not into games, cartoon characters that your dad may be familiar with could be fun. Disney is usually something everyone can find a costume idea from. Then there's Looney Toons
, and Pokemon
My suggestion? Sit down and figure out something you're all interested in, and plan from there. You don't really want to spend a ton of time and money on a costume that your heart's not totally into. I have a video that I just posted today, ironically, on my YouTube channel that's all about figuring out who to cosplay. It's a bit long... And I'm a bit of a scatter brain... but you can feel free to watch it, if you want. It's called 15 Cosplay Questions.
Good luck with figuring things out! I hope I helped you somewhat? So glad to see another family cosplay group out there though! You guys have fun!
(P.S. Don't be afraid to message me through my YouTube or Facebook Page if you ever have any questions! I love helping people!!!!!