Rumer Willis

Rumer Willis

Rumer Willis is "blown away" by her younger sister's strength.

The 26-year-old star, who is the eldest daughter of Demi Moore and ex-husband Bruce Willis, is very proud of Tallulah Willis for getting her life back on track following her recent stint in rehab.

The actress told PEOPLE.com: "I've actually been so impressed and blown away by her strength and her openness and willingness. Not only to move past things but also to share herself with other people."

She added: "The best thing you can do for anyone in your life that you know is struggling with anything is just to be supportive and not abandon them. Because I think that's the hardest thing, when you're going through something and you feel alone and you feel like people don't have your back."

The 20-year-old blogger, who spent six weeks in rehab shortly after admitting she battled an eating disorder and body dysmorphia when she was younger, is now doing "really well" and recently celebrated 90-days of sobriety.

Rumer, who also has sister Scout, 23, and spent much of her childhood in Idaho, believes Tallulah's problems stem from growing up in Hollywood.

She said: "When you grow up like we do, everyone has an opinion of you. Everyone wants to put you in a particular box."