My sister has been riding my mare and she just had a foal a month a go will she hurt her?

My mare successfully had a bay colt April 13th and he is about a month and a half old now. My mare was very active before she had it. But i have not done any riding on her since she had it. Some one told my little sister it was ok for her to ride her while i was out of town this week and i was wondering is that going to hurt my mare in any way or the foal? I wont be home for another 2 days and i want to make sure it will not hurt her. If it will not hurt her then i may let her continue to ride her because im sure my mare is enjoying the attention.