Barbara,

This is why I don't do presents. People get stuff they don't want.

Your sister probably spent the $80.00 already (did you check with your bank to see if the check got cashed?)
If she spent it, you can be sure that she put the money to good use.

Once you send off a check, you don't have any power over how the person is going to spend it.
(This is why some people use gift cards--so they can make sure that the other person uses the gift at Barnes and Noble, Nike, . . .wherever.) But I think that gift cards are tacky and bossy.

Try to be gracious and let it go.

She sent you an awful Christmas present. You sent her a nice cheque because you had in mind that she would go through with what you talked about and buy some good software.
She is not going to say anything like that but that is the truth.
You know that you sent a gift from your heart because you wanted her to have the software that you talked about.

Maybe take a break from her until this does not bother you as much.

Mari