I'm guessing that they were abandoned. There had to be a biological father for them to exist, so maybe he left? We know in the 80s series that Vinny abandoned her kids because she didn't have resources to keep them alive, how she knew Dave would care for them, I don't know.
There is much speculation among fans about their relatedness in the current series, but it's possible that their mom mated with multiple males like a typical rodent in all iterations. Or maybe they have different biological mothers.