Hilary Duff is helping her older sister to regain her pre-pregnancy figure.

The 'Chasing the Sun' hitmaker, who has three-year-old son Luca with estranged husband Mike Comrie, is helping her sibling Haylie get back into shape after giving birth to her first child, Ryan, in May.

A source told Life & Style Weekly: ''Hilary knows it can be intimidating to get your butt kicked by a trainer right away, so she's starting off slowly with Haylie. They both love to hike, so the plan is to do that together, with their kids in tow, between workout sessions.

''Haylie has vowed to get her body back by the New Year, with Hilary's help, of course!''

Hilary, who ''knows the post-baby body struggle better than anyone'', previously admitted she felt a lot of pressure to slim down after welcoming Luca into the world in 2012.

Speaking earlier this year, she said: ''I was the girl who looked at moms who just had babies and was like, 'How did they do that?' Everyone was like, 'Nurse the baby, the weight is going to fall off!' I feel like the opposite happened to me -- and I nursed him exclusively for seven and a half months! It took me two years to get in shape. My body is never going to be what it was. I have stretch marks, my boobs are different -- much different! And that's fine and I'm happy with them.''