Shiv's father once said that she was settling for Tom because she feared being betrayed. He was right, and the joke was on her when he turned around and betrayed her, by siding with her father.
Her father betrayed her too. He strung her along promising her the company, just like he did with Kendall and Roman. He was never going to give it to her. She was mortified when that hit her.
Her mother, well she was never really around to betray her, was she? Except for those summers she spent in England. She remembers one of her mother's boyfriends, a very important man in politics, who liked to hang around outside the bathroom when the teenage Shiv was in there. When she mentioned this to her mother, Caroline paused and then said calmly well you can't blame him for that darling, you're an attractive girl. After that she got Roman or Kendall to stand guard in the hall whenever she was in the shower.
Her brothers betray her too. Kendall in particular, he's the one she trusted the most, Connor being more like an uncle, Roman being more of an annoyance. But he grabbed the crown after their father died and ran with it, cutting her out of his and Roman's little boys' club and keeping her in the dark over their plan to tank the deal. So fuck them.
Matsson's betrayal was the final straw, really. He had strung her along too while laughing at her, getting her to betray her family when he never had any intention of making her CEO. When she finds out it's actually going to be Tom, she has every intention of taking them down. It's only when sitting in the boardroom, looking at Roman's bleeding face and Ken's smug expression that she thinks what am I doing?
She doesn't choose Tom or her unborn child over her brother. She doesn't choose to let Matsson win, she still hates him. Let him have the company, let Tom have the job that puts you in an early grave, who cares? She just knows that if Kendall gets it, he'll be even more insufferable and he won't hesitate to stab her in the back again, despite all his promises. And she can't take any more betrayal.
So she chooses herself.