Firstly, welcome to the board.
Now, I AM a Sami hater. I could tolerate her with Rafe and Lucas, but I've always found the character very immature and childish, and also a judgmental hypocrite. My biggest pet peeve is a hypocrite. I DO think Abby and EJ were very wrong in their affair. It was morally reprehensible. Just as Abby was wrong in deceiving Austin and Carrie, and leading Chad and Cameron on without making a decision. But, those were also very badly written storylines.
Anyway, she has also told everyone who blames it solely on EJ that it wasn't just his fault, she is just as much to blame, and apologizes, even understands Sami hating her. The thing is, as some users here said, people don't expect much from EJ, while they know that while Abby makes mistakes, she is a good person deep down. EJ, on the other hand, always tries to change, but fails and does awful things because he is a terrible person deep down.
People expect so much more from Abby, and since her past wrongdoings were so long ago, they believed that she had grown as a person and never would do something like this--especially not with EJ, the older half-brother of her ex and a DiMera. The other thing: people have never cared for EJ and Sami as a couple, and don't really like them as individuals either (besides their families), so why would they be sympathetic towards Sami? Especially since she knew EXACTLY what kind of man EJ was, and how he (and Sami too) have made so many Salem residents live in misery?
The thing I have a problem with is this: Sami went too far. Calling Abby out at the Horton House in front of her mom and revealing the truth to Ben (and probably others) was enough. But threatening sick children and using her aunt in a scheme to get Abby fired was wrong.
Sami is only going to hurt herself and the people she cares about in the end, especially since, while people are disappointed in Abby, she is not going to be the town pariah. Nobody really cares that much to disown her or yell at her. And who is Sami to judge Abby? She's done the same thing, and FAR worse! She's just angry that everyone was right about EJ all along, and that it is now her on the receiving end of an affair. I say it's karma, and while I enjoy all the drama of this storyline, I don't feel sorry for Sami at all.