why like this?

When me n husb went out for few hrs to take break, mil will take care of LO. But once we home, she will nag nag nag. But when i saw my sil (husb'sis) asked my mil to look after her LO, when my sil is back, mil didnot nag, even can smile smile updating sil of her LO' activities etc. Why different treatment sia?? Just ranting out~~~