Custody help

I was wondering if anyone knew what to do with this situation.

I am 19 and still live with my parents my brother 29 just moved back in with his son who is 16 months old he has a crazy ex girlfriend who is dating a drug dealer she does drugs her self she has told me multiple times from doing shrooms, cocaine, weed, and alcohol almost every night she now lives with her boyfriend and has been saying she is coming to take her son and have him every other night I am not ok with this I am like my nephews mom I feed him bathe him watch him 6 hours a day every day and she only visits him once a week for 30 minutes we can’t get full custody of him because my brother doesn’t have the money but I don’t want her to take my nephew I am so scared and don’t know what to do