smoking weed w baby in house

I just visited my sister and her husband to meet my niece who is 7 months old. I found out while there that when the baby goes to sleep, sometimes they will sit in the living room and smoke a joint. They keep both the windows open and put a towel under the baby's door which is near the living room. I personally would never smoke inside at all but they seem sure that no smoke is getting in her room and no harm is being done....with out getting angry, do you guys honestly think there is any smoke getting in there and harming her?