While I do understand your question and in another scenario would probably be able to sympathize with your intention, in this specific case, I don't really care.
Sure, she's family, sure she's hot, but, not only was she behind the attempt to kill you, she did kill (with pleasure) your mother and maybe also your father. She would have seen you die, or even kill you herself, that same night. The reasons why, or even under who's power she was, mean very little.
Unless Uncle has more info that he hasn't divulged that completely turns this around, I can't see any redemption arc here.
About manipulation, is the MC also not being manipulated? Alexis is awesome to him, but, others could call her a monster. How many have her killed? And will kill yet? The families and lovers of all those hunters, that have been erased, can also say that the MC, Alexis and the gang, should all be killed on sight. Did you really choose this path? Or, out of (very well deserved) gratitude for what she did for you, did you follow Alexis and made a choice of sides based on who's nice to you?
Can't Britney have been on something similar? Yes, she could. The difference is that, even if she is your sibling, she's after the people that have given you love and care, while you got nothing but hostility and murder intentions from her.
In my view, and anyone is clear to object, her intentions and her actions are far more relevant than whatever blood ties she may have. Where are those intentions coming from, loses all relevancy when she actively murders your mother and tries to murder you.
Until new info, relevant and very convincing, comes to light, I remain:
#SayNoToBritney
