Does your sister have a copy of the returns? Can she get them if she doesn't? And does she know where he banks?
How long ago did this happen?
I'm asking these questions because my husband did the same thing, tried to I should say but w/the help of the IRS and his bank I finally retrieved MY rightful half.
I too, according to my husband didn't have a job so he felt he deserved it but without me he wouldn't have gotten back as much as we did.
I'm editing my post as I just found out via IRS website that you can request a copy of the original tax refund filed under your name as well as a micro film copy of the front & back of the cancelled refund check. I called the IRS @ 1-800-829-0922, I wrote down every persons name I spoke to, their ID # and made sure I told as many people possible my situation. Then I wrote my letter including them in it.
A further help would be to go to the bank where the check was deposited first and see what actions they're going to take in order to help you. I spoke to supervisors of the bank and asked them what they planned to do about this and to my surprise they actually froze the monies pending investigation. I added this in my letter to the IRS as well yet at the time I wasn't aware that the monies were frozen. I found out by my freaked out husband.
In the end I found out that my husband never signed my name, it was a bank error, so the bank had to refund me my money.
Oh, be prepared they will try to tell you that they can't guarantee you'll get your half, but you just keep reminding them of how you have no account with them, therefore they have no right to be holding your money. Persistence, persistence,persistence and in the end I'll be praying your sister gets her half.