If the accounts were in your name(s) you did not inherit anything.
If his name is on the account it seems he is a one third owner with a step-up-basis at time of death.
you both inherited his third which will have the high basis.
This is my opinion, I'm not a tax advisor.
P.S.
If your names were on the account, why did you let such large losses accrue ?
Or did you find out about this account after he died.?
@Slappy McGee
