Pauls other sister

I do not remember ever seeing her. I only remember seeing Debbie.

Was another sister mentioned?

Yes, she was, but don't blink or you'll miss her. In S2E3, "Bedfellows" (when the family visits Burt in the hospital) the actress Randy Graff is billed as "Sharon, Paul's Elder Sister."

She's also mentioned in S6E15, "The Second Mrs. Buchman." When Burt is comes up to bat in his senior softball league, the announcer lists his family members. We just saw that one, so I'm aware of the mention, but I suspect she was mentioned in a few other episodes as well.