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.
First of all, we ask Allaah to reward you for being keen on being kind and dutiful to your mother and we ask Him to enable her to repent.
Gambling is one of the grave major sins and Allaah considered it as a defilement from the work of the devil; Allaah Says (what means): {O you who have believed, indeed, intoxicants, gambling, [sacrificing on] stone alters [to other than Allaah], and divining arrows are but defilement from the work of Satan, so avoid it that you may be successful.}[Quran 5:90]
Therefore, among the best acts of being kind and dutiful to your mother is to be a reason for her guidance. The best thing that can help you in this matter is the supplication. Therefore, you should continue to supplicate and choose the best times and circumstances when the supplication is most likely to be answered, like the last third part of the night, in the prostration position and so forth; for more benefit, please refer to Fatwa 88296.
Also, you should advise her in a gentle and soft manner and choose the times when she is most likely to accept advice. If you think that a scholar or a righteous man could influence her, then you may seek his help so that he would suitably advise her.
For more benefit, please refer to Fataawa 89633 and 88120.
Allaah Knows best.