What the Ryders and Their Sidekicks Were Doing During the Events of Mass Effect 1

Bro - being trained for the AI expedition.
Drack - slowly realising that after many centuries of living its time to move and do something different.
Vetra - teaching Garrus some martial arts, then teaching him the meaning of flexibility.
Liam - working as a police officer with good records. That probably got him recruited for AI.
Cora - Now this is hard guess, since we know less about her than about Liam.
Sis - ... being trained for the AI expedition like her Bro.
Peebee - probably rebelling against her parents and tagging along in secret for AI expedition.

Would be indeed funny, but also it would feel soooo forced and far fetched... almost blatantly as a pure fan service.
Still... this is Bioware, so I put that prediction for her above:)

I did wonder so as well, in fact.

And I heard that hand-to-hand is indeed coming, so maybe she'll be great at martial arts as a fall back during combat, or even just story-wise?

Your avatar will have the simplest of reasons imo. She's Turian and was given the order.

Not Confirmed:

Drack is son to Nakmor Kesh, Nexus Superintendent. Only child she sired, her relationship with him is so strong, Drack gave up being Shaman to his clan so as to go with her.

Scott - As someone already said; probably training for the Ai program. Or serving in the Alliance? Or in school? Mac Walters did say that Scott/Sara are in their mid-twenties (so probably late 23 to early 27).

Sara - Probably doing the same as Scott (maybe one went to school and the other joined the Alliance).

Drackula - He was either a Warlord/mercenary hired for security or a Shaman trying to establish a future for the Krogan elsewhere. Or both? Or he has a Master's degree in archaeology or something (thus the bones).

Vetra - Probably doing her time in the Turians Military (her sleek armor and the holographic visor make me think she was a scout). That or training for the Ai program.

Liam - Probably busy being kicked off the force for being a loose cannon. Or killing a Batarian slaver with diplomatic immunity (revoked it with a bullet of course). Or he was busy training for the Ai program (or training security personnel and members of the Pathfinder team).

Cora - Well... She is wearing the white and blue uniform of the human Ai personnel. But she is not wearing the same space helmet as Scott/Sara/Jien/Alec/Liam so... Maybe she was a combat pilot (the Arks and Nexus video mentions combat pilots protecting the Arks), and the Alliance fighter pilot we saw at the end of ME3 had a helmet similar in design to hers.

Peebee - Busy being a hipster.

Daddy Ryder was probably doing with them the song
"I'll make a man out of you"

Also Ryders are supposed to be in their 25's so they would have been born around 2160.

SAM could be a stowaway if he's an ai. We don't know much about him either. Also krogan and asari can pull the feat since they are long lived species.

Edit. Oops. My info about SAM was outdated. Hadn't seen that interview with Fabrice Condominas. So forget what I said about SAM.

I think in that video Mac said he thought about mid 20-ish. Which could pretty much mean anything 23-26? But he when he said this it seemed like he was just guessing, so they could be earlier 20s.