Well so, who is the other person that Sun is thinks is responsible for Jin's (apparent) death?

It seems clear that Jack thinks it is him, by what he started to say to Ben.

Other guesses would have to be made based on what information Sun has.
OR What information we think Sun has.

So what information do we as viewers of the show know that Sun has?

It seems by her visit to Widmore that she knows that it was his frieghter, his mercenaries, and his plan that ultimately ended up in the frieghter explosion.

So Widmore could be the other person she blames.

Since Locke(as Jeremy Bentham) visited Jack, Hurley, Kate, and Walt. Then it is plausible that he could have visited Sun and told her the events that led to the freighter explosion.

So Ben could be the other person she blames.

As far as Sun blaming herself? I don't think so. She doesn't take responsibility like that.
I think back to the scene of her as a child in The Glass Ballerina where she wouldn't admit to her father that she broke the glass ballerina even when her father said he would fire the maid if she didn't admit to it. And I know she was just a child and kids lie to avoid punishment, but I think the scene was meant to show the kind of person she would grow up into.
I think she should blame herself for Jin's death because she told her father that she blames him because he(Mr. Paik) was the reason that Sun and Jin were on flight 815 in the first place. And by that same reasoning Jin wouldn't be working for Mr. Paik if not for some decisions that Sun made. anyways....

I think she blames Widmore. And she is working with him only to get her revenge on him.
Keep your friends close, but keep your enemies closer.

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Doctor playing golf. Woo, boy howdy, now I've heard everything. What's next, cop eating a donut?