I think that Melissa and Ralph are married and that Bert is a brother/brother in law, y'know the kind that eats all your food and crashes in your house "for a week", but never leaves.
I also think Bert is much younger than Ralph and Melissa.
What does everyone else think?
Turn a frown upside down...and keep frowning.
It's pretty safe to say that Ralph and Melissa are married. But that's pretty much all we know.
In the episode Bentley appeared for the first time it was made clear that he is related to either Ralph or Melissa. In the episode when George (Bentley's father) moved into the Evergreen Forest he and Ralph talked about their childhood, so I guess Ralph is George's brother and thus Bentley's uncle.
As for Bert, there was an episode showing him going to school with Ralph, Melissa and Cedric, so he might be younger but not much. He's an old childhood friend of theirs but not related by blood to either of them. That's what I think.