It is easy to forget mum when a baby comes along and steals all the attention! But the happier and healthier a mother is, the happier the whole family will be.
Take things slowly and be subtle. Why not make a regular date to go out together for a brisk walk and a chat?
Pushing a pram while walking briskly is a great cardio workout, and she will get a Vitamin D and a serotonin boost from being outside. Go with her and keep the pace steady.
Why not go for a swim together? Anything that helps you have some fun time together while incorporating gentle exercise should get your sister off to a great start and when baby is old enough they can join in too.