Can I Buy Minutes For Someone Else's Phone?

I don't have a Net10, have never used one.  My sister has one, and I'd like to send her some minutes, as she has an emergency situation going on, no money, & is low on minutes.  How can I do this?  I tried to contact Net10, but to email them with a question, it required a phone serial number, even when I checked that I did not have a Net10 phone.  I also need to do this in such a way that I do not have to give her my creidt card number, or have my credit card number accessible to her for future purchases (she has untrustworthy family members, unfortunately). Any help would be appreciated.