Children nowadays live in very good conditions and have a very comfortable and clean environment at home. They will give their children the best living environment. It is also because of these superior living environments that some moms develop a kind of cleanliness fetish.
She may think that her in-laws are old and not as healthy as they were when they were young, and they may have some chronic diseases, so she worries that her child will develop some illnesses if he sleeps with them.
Particularly in-laws whose families are not so well off or elderly people living in the countryside, the era and lifestyle they experienced is unacceptable to modern people.
The main reason why children are not allowed to sleep with their in-laws is because of their health concerns. There are some older people who still give off a smell of old age on their bodies which is also very hard to accept.
So most old people may not be so clean and hygienic in the eyes of young people. How can their beloved baby sleep with such old people. It will make them worry about the health of the baby.