Which is likely to happen. Most people don't find there life partners at 18; drunk; and at mardi grai.
If they had gotten married right away, I would agree, but I'm more cautious since they seem to have taken a little time. I'd think the big danger would be the risk that Rick was drawn to Lisa by her similiarities to Karrin, which might be the case. Alternatively, there'd be the risk that there was an element of 'screw you Sis' in Lisa's decision to screw/marry Rick. Or both.
On the other hand, they might grow into Charity and Michael. Too soon to say.