A skunk in a rose patch doesn't smell any sweeter, even if you name it Rose (pretending to think like Bob).

"Did the stepped up basis from Dads date of death get factored in here? Is it a Community Property state?"

From Dad's death, only the Mom has any step up potential and only for that one-time event. The kids on the deed with Mom, when no one died since, means nothing steps up for them. The kids have a 1/4 of Mom's basis at the time the deed was split. She gifted them that value at the time, and now has a 1/4 share, as well. That's the sequence:

Dad dies

Revalue based on Common Law or Community Property, for Mom's basis

Split Mom's basis 4 ways.

Subtract each 1/4 from $100k, and each kid has taxable gain.