Pete Wentz and Ashlee Simpson-Wentz may be on their way to becoming first-time parents, but Ashlee’s older sister has no doubt the sort of mother her sibling will be.

“She’ll be amazing, absolutely amazing,” Jessica Simpson, 27, told PEOPLE in Las Vegas Saturday at the grand opening of Palms Place.

Jessica, who is dipping into the country-music scene, also says that her sister’s relationship with the Fall Out Boy bassist helps inspire her own music.

“They’re just the cutest couple in the world,” she says. “They are adorable. Anybody would wish to be them and you can only wish them well. Just being around them inspires love and everything I sing about.”

At the Vegas event, newlywed Ashlee, 23, never strayed far from her sister, as the two hung out in the VIP area and took photos together – several with Jess’s hand on her sister’s stomach – while Pete Wentz remained only a few feet away, playing DJ at the event.

Came the end of his two-hour set, Wentz, 28, thanked his “baby mama.”