All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
First of all, it should be noted that if you have no evidence that this money was obtained by your mother from a forbidden source, then it is permissible for you to benefit from it, as in principle the money is lawful.
The same thing applies if her money is obtained from both lawful and unlawful sources.
However, if it is confirmed that she obtained this exact money from forbidden sources, then the matter is one of the following:
1- In case this money belongs to a specific owner, then it should be returned to him as explained in Fataawa 89762 and 81223.
2- If this money does not belong to a specific owner, then one should get rid of it by spending it in charitable fields.
Finally, it should be noted that this mother should be advised to repent if it is confirmed that she had taken other people's money without right. You should also repent if you had taken this money while knowing that it was taken from forbidden sources.
Allaah Knows best.