It could have something to do with the fact that she is in a cot at Grandma's??

My DS is 18 months old and his sleep pattern is as follows...
8am: Wakes
11am-1pm: Nap
4pm-5pm: Nap (some days he sleeps the whole hour, other days 15 mins - pretty sure he is in the process of dropping this)
6:30pm: Bedtime and he goes down without any issues.

He hasn't woken during the night since he was 5 months old so we have never had an issue with that.