Sleeping on stomach

Can someone please tell me I am not overreacting. My youngest son was born in December so just a couple months old. Almost 3 months on the 20th of this month. My sister watches him & informed me she lays him on his stomach when he sleeps. I freaked out. I told her I wasn’t comfortable with that. She’s like well I’ve talked to doctors who said if they sleep better blah blah. My cousin is a nurse & said no doctor would tell you to lay a baby on their stomach. She said if he rolls on his own that’s different. So today I go pick him up he was on stomach laying on a burp rag. I’m like wtf he doesn’t have head control still. If he was suffocating he would just lay there or if he threw up he would lay in it. It gives me bad anxiety I just don’t understand why she would do that. He is still so small. Does anyone else do this? I’m not judging if so it just really makes me nervous personally.