Sun and Jin are the type of characters tha can be interesting as we progress in the season, but at this moment, their situation is not very interesting.
Sun flashbacks does not show too much. We know that she wanted to leave her husband, but she didn´t.
Contrary to some moments in the last episodes and since the beginning, the impression that Jin give us, is not the same in the flashbacks. He seems to love his wife and he is capable of doing what is necessary to make her happy.
Sun revelation is not big or something special, still can caught some audience by surprise.
Her flashbacks are incomplete, since we don´t know what type of job was Jin doing for her father and since they are a couple, Jin flashback will reveal that.