What affect does smoking pot during pregnancy have on the fetus/child?

My 28 week pregnant "almost" sister in-law came to visit and we sat in the backyard and she lit up a cigarrette and it made me feel a little uncomfortable (cuz I hate smoke and I know what it can to do unborn babies) but i thought "heck, she's a grown up and it's none of my business". BUT then she lit up a small pipe and took a couple hits and told me that she just couldnt' break her pot habit and said that it helps her relax. Well, geez, I am 25 weeks pregnant and stopped drinking alcohol, I know I can stop for 9 months....and a nice warm bath and some stretching helps me relax. Is there any research that proves pot harmful or not to a fetus? Because I know people who smoke it for medical reasons. And, should I have the right to say something to her? I'm sorta a goody-goody, I don't smoke or smoke pot ever, and this is my 2nd pregnancy and both times i went very healthy. I don't want to step on feet but i feel that she is being selfish. ??? Thank you.