I think it would have been a better plot to find out in Nemesis at the end that Sela was Tasha surgically altered and mentally tampered with to think she is her own daughter.
What's ironic is that the TNG porn satire produced a few years ago actually had a good storyline: Alternate Tasha wasn't dead, just in suspended animation. She was then released and returned to the Enterprise-D, where she interacted with the crew even though she was from the alternate Klingon war universe. Then it turned out that Sela had "programmed" Tasha like Shinzon programmed B4 (to get information about the Enterprise without knowing she was actually doing it). When the truth was uncovered, Tasha ends up dying, but at least she died surrounded by her friends.