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.
May Allaah increase your keenness on doing good and seeking lawful earnings. You should know that it is permissible for you to benefit from your mother's property if she gives you something from it, be it food or anything else, and this does not affect the validity of your prayers or the acceptance of your supplications.
Suspicions that some of her money might be unlawful are of no significance, i.e. there needs to be preponderant evidence. If your mother misses days of work without excuse, then she must ask her employer to pardon her for those absences. If her employer pardons her, then her entire salary is lawful for her, otherwise she has to reimburse her employer for the days she was absent without permission. For more benefit, please refer to Fatwa 96265.
If it is true that she was absent from work without permission, then her money is mixed (part lawful and part unlawful) because she does not have right to receive pay for the days she was absent. We have previously clarified that it is permissible to deal with someone whose money is mixed as long as the unlawful money is not the dominant part, as in Fataawa 81560 and 87211.
Allaah Knows best.