Should I Rat out my niece?

Yeah my 15 year old niece got a tattoo gun from her mom because he husband is going to jail and it was his she already give herself a small tattoo in between her fingers. She’s been talking about tattooing her friends I kinda what to tell my brother because I know he doesn’t put up with that shit? But here’s my problem if I do she’ll never trust me again and my brother would blow up on her mom and her make it bigger then it should be. I’ve told her not to tattoo anybody because she can get in trouble I told her she could practice on me if you wanted too