Mackenzie Rosman — who plays sassy little Ruthie Camden on 7th Heaven — is used to facing adult issues. The 13-year-old actress already has confronted everything from racism to death on the WB's top-rated family drama. However, the show will really hit home when Rosman's stepsister, Katelyn Salmont, guest stars to talk about her real-life struggle with cystic fibrosis.

"We've wanted to do this for a while," Rosman tells TV Guide Online, so she made a pitch to Heaven's executive producer, Brenda Hampton. "Mackenzie came to me, notes in hand, and told me all about CF and asked me if I would do a story for her sister," she recalls. "I thought it was very touching."

Katelyn, 16, has struggled with CF — a genetic disease that shortens the life span to approximately 20 years — since she was 2 years old. Despite the stress this ailment places on her and the family, Hampton says Mack and Katelyn are "just the nicest, most normal, polite, smart, sweet children."

In Monday's episode (airing at 8 pm/ET), Ruthie befriends Katelyn while horseback riding, a sport their family is passionate about in real life. For Rosman, acting with her sib — not to mention stepdad Randy, who has a cameo — was an exciting opportunity.

"It's absolutely beautiful," Hampton previews. "It's shot with some scenes of them riding horses at sunset, and you can read between the lines, just looking at the two girls riding together. Katelyn is an amazing human being. I dare anyone to watch it and not get a little tear in their eye."