What is the ruling on me taking my sister's votive money, which she entrusted to me to donate as charity, given that I am in need of it as a student of knowledge?

If your sister specified a recipient for the vow, and you are not among them, then it is not permissible for you to take from the money, and it must be spent on what she vowed for. However, if she did not specify a recipient, or if you are included in the specified recipient, then it would be better not to take from it to avoid scholarly disagreement, even though the preponderant opinion is that it is permissible for you to take from it in this case.