In a Jan. 25 episode of the TV Land series (airing at 10 p.m.), the actress, 32, reprises her role as Wendie Malick's daughter Emmy, and disappoints Valerie Bertinelli's character Melanie when she announces that she is engaged to her son Will, played by Joe Jonas. (See clip, below.)
"Our moms are shocked to find out that we're engaged," Hewitt tells PEOPLE. "I'm the wrong person for him because his character is really nice and down to earth, and mine is ridiculous and rude and has all the wrong priorities."
Explaining that the ladies are "pretty upset" about the impending nuptials, "Our parents in the episode try to convince us we're not right for each other," Jonas, 22, says.
Eventually, Malick and Bertinelli leave their kids alone to figure out their incompatibility by themselves.
"At that point, our relationship unravels," Hewitt says. "He sort of figures out who I really am, and I figure out that he's just not as exciting as I thought he was. And then we end up not together at the end. It's just sad."
Adds Jonas, "We eventually realized it's not best."