Incest?
There was one scene where Ma came into the room and said she didn't want to sleep alone that night. One son (I forgot his name) was unwilling so Herman got up and went. It really looked like she wanted to make love with him, and that it wasn't the first time something like that happened!
I'm very surprised nobody else has mentioned this. Even if it was platonic and she just wanted comfort, it's a little unusual for a mother to want to share a bed with her grown-up son.