Have you just started bringing her to your mum-in-law's place? My suspicion is that as she is in a new environment (ref to your mother-in-law place), she is still not used to it and so she stays awake in the afternoon. This change might take a while before she goes back to her normal routine; as you know each baby behave differently. So a little bit of patience is needed
Next, do you give your baby girl a bath or cleaning up before bedtime? Try cleaning her up once you brought her home. Some babies needs to feel refreshed and relaxed after having their body cleaned. However, do try not to bathe her too late! Just in case she is wide awake after the wash, she may not want to sleep.
Hope this helps a little.