I don't think saving her sister's life can ever have been not worth it. At the time it was only a question of one life or death, and we've seen with Garth that it can be controlled. Kate hasn't seen that but she knows she herself is managing being a wolf. It's not on Kate what Tasha went on to do, any more than a paramedic is responsible for the mass murders later committed by someone they've resuscitated. In the same way, Sam isn't responsible for Lester's selfish, murderous, soul-risking life choices. Incitement is wrong but he's guilty of his own misdeeds, not Lester's.
I can't help but feel for Kate and Tasha's parents. One daughter disappears, then the other is involved in a semi-fatal car accident, miraculously recovers, and will then be found stabbed to death (unless Sam and Dean took the time to burn the bodies.) Even in that case, it would mean both daughters had disappeared separately. And if Kate was seen interacting with Tasha at the hospital or later, she might be the main suspect in Tasha's disappearance.
Tasha should have known: if you're going to call someone Mary-Kate and Ashley, remember that their sister is the Scarlet Witch.