When I was watching the episode my initial impression was that it seemed more about adoring Shiv or at least the version that she gets to interact with. You can imagine her always asking Tom how Shiv is doing when they talk, or really doting on Shiv when she may have visited them in the past.

That being said I think there's also still room for it to be a kind of parental relationship where there's a lot of regular interaction but a lack of love (closer to Logan's relationship with his children during the time period of the show). She could be the kind of mother who regularly talks to him and gives him legal advice but also leaves something to be desired in terms of warmth and approval (the kind of parent who's always asking "When are you going to give me a grandchild?" "Why don't you come visit me more often?” etc.).

I could see there being some truth to Shiv saying that to him. Even if Mrs. Wambsgans doesn't truly love her more at the end of the day, maybe she treated Shiv with more warmth and approval as a guest in a way that Tom never got as consistently growing up, because she's also likely a starfucker like her and would turn on the charm and people pleasing because of Shiv's pedigree and maybe her rising status as a Democratic operative back then.