Shosh can't blame Jessa for her decisions in the past, but I think she was just venting and needed to tell her off, regardless of how much sense she made. Jessa always wants everyone to feel inferior, boring and uncool and she has done that to Shosh in particular. Shosh was younger and impressionable and it's no wonder she used to drop everything for Jessa. Now that she's more independent, Jessa can only deal with her cousin's growth and interests by mocking her. If she really cared so little, she would not show up uninvited to an event that is the symbol of everything she despises.
The one thing that is a bit unfair is saying Jessa "stole" Adam. First, that's not a thing. People cannot be stolen and Adam pursued her very intensely. Second, Adam and Hanna were broken up. However, she absolutely should have been more honest about it. At the end of the day, he mattered more to her than hurting her friend's feelings. She chose Adam over Hanna and should not be surprised that the entire group of friends would have a reaction. Hanna can be insufferable, but she's less of a flake and more loyal. They have good reason to side with her.
Shoshanna needs to be more self aware in regards to her insecurity about success and not blame it all on someone else. This has been a theme with her forever: when she graduated late, when she couldn't find a job, when she left for Japan and couldn't handle coming back. But, I think she is taking the right step removing Jessa from her life. They will always be family, but they don't have to be friends, especially if having Jessa around only ever makes her feel bad.