She is staying with you for Christmas. It is so lovely to wake up to a stocking on Christmas morning. It is so lovely to open it and share that joy with children.
I tell my children that Father Christmas just can't provide for all the adults too (caveat that there may be some very special parents and grandparents that Santa does stop for), but the majority of adults go without. So that is why it is OK to buy adults a stocking too, to let them join the fun. The MIL is usually quite hard to buy for, so we always have a few stockings in our collection with a mum-in-law in mind.