To all those who say things like "Her baby, her choice" - do you apply this in all situations? So whatever unsafe or unkind child-rearing practices someone might be following whether through ignorance, carelessness or deliberate malice even, you believe the parents" autonomy is sacrosanct and must be prioritised above the child's wellbeing?
I read this view quite a lot on here and I find it chilling, to be honest. It's a is the child isn't a person in their own right, but somehow a piece of property belonging to the parents for them to treat as they please......
OP, my view is very different. I think you absolutely should say something, and to be honest I am struggling to see why you find the prospect of doing so so scary/off-putting. These are your family members - you know, love and care for them, and they you. They will obviously understand that what you say comes from a place of love - why would they be up in arms about it, never speak to you again etc? Do people really react so strongly if somebody questions something they are doing, or gives advice? It must be an exhausting way to live if so.
Simply say that your understanding is that babies are less safe sleeping on their fronts, and have they had any advice regarding this. Presumably she will either say "Yeah, I asked about it, it's fine for XYZ reason" or "I wasn't aware of this as the baby slept on her front in hospital, but thanks for flagging that, I'll check it now".
She presumably loves her baby and wants it to survive, so why on earth would she be angered by advice that makes this more likely?
Just to add, my baby used to sleep on her side, when she was ever so tiny (she was almost full term but born on the 0.6rg centile - no idea why as she is average height now as a teen). From a couple of weeks of age she could somehow wriggle onto her side. (She was a very squirmy baby - she wrggled all the way off the bed once - most terrifying and guilt-inducing moment of my life). I told the HVs who said that once the baby can move position you have to let them sleep in the position they choose, as the only alternative is to stay awake all night moving them back into position......