Ridiculous Guy
Do you know that for certain though, that she wouldn't be miserable with this Harry? I realize he's your friend but perhaps that's making you biased in who you feel your sister should be with.
I get it. You want the best for your sister and that makes you a good brother but there also comes a time when a brother needs to step back to let his sister soar to happiness, but to always be there just in case she stumbles or falls.
The best advice I can give is let your sister and this Neville know, tactfully, that you're uneasy about them being together and you're worried about your friend Harry's feelings on the matter. Then I would suggest just keeping out of their business. It's a difficult position to be in, caught between your friend and sister, so I would suggest just stepping back and keeping out of it.