To be honest, I can only see them hooking up when they're older. However, for the sake of recognisability...
As for the "Why not?" It kinda references a lot of other notable pairings on DA. As in "If Bart and Sideshow Bob can be seen as a couple, why not Sherri and Lisa?"
Besides, it just be me, but I can totally see Lisa being a lesbian... it's more likely than Bart being gay at the very least. As for Sherri, I have my suspicions... although she'd be bisexual if anything...
Lisa and Sherri (Terri too) belong to 20th Century Fox.
Cool, I didn't think anyone out there was shipping this pairing! It makes sense to me, as Bart and Terri get shipped, to pair up their siblings together too.