In hindsight, the revelation that Linda and Hillary are romantically involved shouldn't be too surprising. Linda has been a surrogate mother for the Foxmans for years, and this only makes that informal relationship official.
Linda fills the traditionally maternal role in the Foxman family. While Hillary is a loving mother, she couldn't be described as "traditional" in any way. Judd even directly says that Linda "mothered [him] as much as [his] own mother" (8.36). We can see evidence in the way that Linda and Judd interact during the shiva. It seems like he always turns to her when things are darkest.
We even learn that Mort was aware of their developing relationship and approved. That's a pretty selfless move. But if anyone deserves his blessing, it's Linda, who's proven her love and dedication to Hillary and the Foxman family many times over.
But there's something really sad about Linda. At one point, Judd questions her about how she manages to stay content:
Linda's smile is sad, ragged, and somehow beautiful, the aching smile of the long-suffering. "You learn not to think about what might have been, and to appreciate what you have." (8.33)
What kind of suffering do you think Linda has experienced? What does she wish might have been?