I don't know how iTunes Match works but I know for sure you can create iTunes accounts with gift cards as I've done it many, many times. The only thing I can think of is that if she uses the same computer as you with the same login (i.e.: she doesn't have her own user on the computer but you all share the same) then her music will end up in iTunes along with yours. I'm assuming that iTunes Match would only match music that belongs to the account holder if this were the case but I have no clue. This is the only potential gotcha that I can think of. If she's got her own computer or uses a separate user account on the 'family' computer then you're set.