I guess it was a kiss between Huey/Louie/Dewey and Webby. Huey's, Louie's and Dewey's mother is Della Duck, but their father is unknown even in the original comics. I guess they think that Webby has the same father as they do, or something like that. Her parents are unknown and she doesn't even exist in the original comics.

I have no idea why she would be related to them, but apparently the developers of the show have said that there will be no shipping because they want to keep all the kids like siblings to each other. But it's "like siblings", not real siblings.