@Azapuela
It is not. However, him being her father is a bit different since both are living currently. For one, she could be used as a hostage, or as a way to "hurt" her father (if he cared about her that is). It is not a great idea, as it is not necessarily going to accomplish much, and she could be innocent of it all, but it is not necessarily incorrect either, since she is a part of the household and did nothing to stop MC's abuse (not that she could necessarily have any impact on it even if she did try to). But if it were her great, great grandfather who has been dead for the last 50 years, then she is completely innocent of being involved with his abuse if the practice is no longer presently done.
Also, she did attempt to kill him, twice even. So he has more than enough of a reason to do so, if he had chosen to kill her. He can justify it with "I have to kill her because she will just try to kill me again, and I cant trust that she is telling the truth if she says she wont try to kill me anymore, because she has already tried 2 times now".