Legal Question in Wills and Trusts in Pennsylvania

If its not in your mother's name then its not an asset of hers and it does not go into her estate unless the executor can prove there was some kind of fraud or wrongdoing by you (undue influence) which caused your mother to do this. So I would not be giving your brother any money on just his say so.

Did your mother have a written will and did the will make mention of these funds or the fact that mother made a gift to you of the funds? If so, what does the will say?

I suggest that if your brother believes these funds to be part of the estate that he consult a probate attorney to advise him on this. If the probate attorney believes the funds can be reclaimed then he needs to send you a detailed letter explaining why after 4 years this can be done. Then you will need to take the letter to your own probate attorney and pay to have it reviewed and make a response on your behalf.

Just telling someone what you think without knowing more is not enough. An attorney will have the opportunity to review all the facts and circumstances and advise if the money can be reclaimed or not. I don't know the basis for your brother's assertion and neither would any attorney here just based on what you posted.