marijuana while breast feeding

My husbands sister told my husband and I that their other sister is smoking marijuana while breast feeding her 5 month old daughter. She smokes everyday multiple times a day. She also smokes around her daughter and her two other children. She will smoke in her car than “air it out” and have her children come into the car afterwards.

My husband and i do not have a good relationship with her at all. We don’t agree with her decisions and we don’t think she is a good mother. Not only the smoking and breast feeding situation but a lot more things as well.

I’m wondering what could my husband and I do? Should we tell somebody about smoking marijuana and breast feeding? If so, who should we know.