Your mother already got stepped-up basis on your step-father's half of the house. She doesn't have to do anything to get it. It's automatic. So her basis is now $150,000 on her half, plus $400,000 on your step-father's half, which is a total of $550,000. If she sells the house for $850,000 her gain will be $300,000. And as tagteam said, the stepped-up basis does not expire.
Don't forget that she can add to her basis the cost of any improvements that they added to the house. Selling expenses, such as a broker's commission will be subtracted from her gain.
It's not clear what you mean by "transferring ownership" to a family member. If the family member does not pay for the house, the exclusion will be lost.
