I think Ruby has a history of mental instability, and this would be another example of that so she would lose her kids. She's texted Billy twice -- once 12 years ago and once five, and my guess is the five was the breakdown Laurence insinuated. I don't think she'd lose her kids, but she might lose custody.
I don't think Billy pushed Fiona, and Fiona is the bad guy in this. She did stalk them; she did extort and she did steal. While she didn't deserve to die, she's not a victim of Billy and Ruby. I think if they had chosen to talk to the police immediately they might have gotten away with things, but not without a lot of difficulty. If the taxidermist doesn't say anything, though, i think they can get off. Plenty of reasons for there to be Billy and Ruby's DNA on Fiona, and if the train does not have video of them jumping, or if the train doesn't have video of them getting back on ... as long as there's no real witness to them not being on the train, the story might work.
Except for cell phone tracking. I don't know how that all works.