I dont really think its due to your mom-in-law, as our baby also has this behaviour without anything similar hitting him.

Its not recommended to sleep the nite thru on the tummy, due to risk of asphyxiation (spelling?).

BUT its damned good to prevent baby from getting scared, as most of the limbs become sort of tied down, it mimics the tight constrains of the womb.

So sometimes when our baby gets fussy or wakes too easily, we will flip him over to the tummy. Then when he falls deep asleep we'll turn him over again. Major effort I know.

However if you're confident your baby won't suffocate facing downward its a great method to prevent the scares. What you can do is use a pillow that's a bit bouncy and will not easily fill up the gaps. Using a pacifier also ensures there is gap between the nose and pillow.

Another method to prevent scaring is to swaddle the child in a blanket or cloth diaper. We used this method from 3weeks to 10 weeks while the baby still weak at the arms. If done correctly it almost ensures our baby will be quiet for a few hours. Need to make sure they are not too hot though.

All the best and don't worry too much.

One important thing if you're trying tummy sleeping...you need to pujuk the baby to sleep first...you can't expect the baby to turn over and suddenly fall asleep!