Here is one way of looking at it:
Laura and Maddy were very close when they were growing up. Maddy often used to stay at Laura's house during the summer holidays. Laura and Maddy became very close and could almost read one another's thoughts. Beth did not come round so often, and consequently, did not get to know Laura so well.
When Laura died, Maddy was very upset and came over to the funeral. Beth did not really know Laura that well, so Laura's death did not affect her, and she did not come to the funeral.
Does this sound plausible?
I don't know Fred, I'm putting myself in Beth's shoes when I write this, my sister's/brother's daughter had tragically died, it doesn't really matter how well I knew my niece, what matters is that my sister/brother lost his child and needs my support. I just couldn't send my daughter to confort them and just give Sarah/Leland a conforting phone call and that's it. But that's just me, since we never knew Beth, we don't what kind of person she was.
And since Maddy mentioned her father died and her mother never fully recovered (Ep 9 missing scene), maybe Beth couldn't deal with another close person dying and just stayed away.
Yeah, I know, I'm making such a big deal for something that's not really that important, just wanted to know what you guys thought about it, we're all peakers here right? :)

"You know, Agent Desmond, I figure this whole office, furniture included, is worth 27,000 dollars"