If I am not mistaken from the earlier chapters...irs mentioned that the new family she was with abandoned her as well after knowing that she has a son befoer them and abandoned him. That's why she's alone and had nowhere to go and maybe since she knows that Dojin is well off and asked for help. Actually I also do not understand Dojins action here. Maybe he has a soft spot for her because they really look a like.
But yes you are correct, Hye-Sung is the biggest VICTIM here and he deserves justice!

While I get it that the story must progress, show character development, tie loose ends and give closure. Hye-sung also needs justice. He was used as a baby trap from the father, married out of obligation. Resulting for him to not loving her and Hye-sung. From LIAI they depict some sort of abuse thats why she ran away and left Hye-Sung. I do not understand why have you left him with a father that has no love for your son. Hye-Sung suffered neglect all his younger life and had to fend for himself. Hye-Sung has really come a long way. Im not crying (/TДT)/