Correct usage:
1. My dad's accounts are named after my grandparents.
2. My dad's accounts' names are Virgilio and Maria.
Between the second d in dad and the s in dad. dad's mug.
As a plural, for more than one dad; no, does not need an apostrophe. As a possessive, as in the item belonging to dad, then yes, it does need an apostrophe.
Yes. The race belongs to the mums and dads so a possessive is required. "Mums' and Dads' Race" would be correct.
In both Dave's and dad's. Dad belongs to Dave and group belongs to dad.
Because the apostrophe follows the plural noun dads, the possessive form means that it applies to more than one dad. Note that a capitalized Dad (a specific person) must be singular.
He would be your second cousin.
Second couins
John Adams, the second president
Your second cousins because they are your mums or dads cousinx
Second cousins once removed.
pogo
first, there is "your mom." second, there are "your two dads." Lastly it could be in your ****.