I am the unwilling tech support for MIL, who is less than nice to me. Biggest problem, I don't have time to drive to her house all the time, especially for a one-day offer she wants to buy. Another issue, she just got her first email account and can't remember the email address or the password.
Ah ha. This is one situation I've been in, except replace MIL with sister. I'm still working on growing my polite spine, but in the mean time, me moving 700 miles away helped quell her cries of needing technical support.
Every once in a while she'll still call with a question, but if it's something that I don't have time to help her figure out (or quite honestly, just don't want to try and help her figure out, as horrible as that sounds), I say, "That's something I would have to see in order to know what's going on." Maybe in your case, you could rephrase it to, "I have to (insert random errand that will take a while to accomplish) so I'm unable to come over, and that's a problem I'd have to see in order to figure out what's going on." Then beandip, if possible.
If she's at least somewhat proficient with using Google, it can make a nice tech support agent as well. Suggest she Google the problem.
Or just use call screening. That came in handy a few times when I didn't want to play bench tech for the day, hehe.