Her Torenthi kin don't seem to have cared about her or her brother for most of their lives, not to support them when their uncle died or when they were beseiged and captured. Torenth may have known they were being held in comfortable captivity, but remember how even Javan didn't know what had happened to them after that. Sighere's family seems to have kept her marriage to Hrorik rather quiet until Javan's coronation.
As for what made Miklos kill her in 928: wasn't it earlier that same year she'd finally agreed to a meeting with him, where she reiterrated her loyalty to Gwynedd and the Haldanes, which led to Hrorik's death at Cullicairn? I'd agree that her "betrayal" of her race was a large part of Miklos wanting her dead, but I think her refusal to support Marek was a big factor too.