Yes, she can certainly come and stay with you.
Travelling during Iddat is not prohibited; staying at home during Iddat is no
directive of the Sharī‘ah, as is generally understood. It needs to be
appreciated that there are two basic objectives for this waiting period (Iddat)
for a widow:
1) To ascertain whether she is pregnant or not. If she is,
this waiting period extends to the birth of the child.
2) To provide her with some time to adjust to the situation
of her husband’s demise.
For ladies who have passed the child bearing age, as in
your mother-in-law’s case, obviously the first condition does not apply. As far
as the second one is concerned, it is befitting that a widow, during this
period, should not go about doing things which are against its objectives stated
above. The mourning period has its own sanctity and a widow should observe this
period with solemnity and austerity. Keeping these objectives in consideration,
she can lead her life in any way she wants.