@Doctor
Seju didn't steal the ring - She did. Sungji wouldn't forget something like that. And it's also important we saw her take it off through Seju's eyes. The guilty party(PG rated...I think I can do this)
You should go check again ch53-54.
I think Sungji did forget the ring.
Even if Seju had taken it when Sungji wasn't there ( how it happen?),then Sungji must realized and asked for it (if she noticed).
I've just been reading comments lately but this one caught my attention the most. Seju was sick and sungji was doing tsb and went to sleep. when she woke sungji was gone and saw the ring beside the wash basin on the side table. And this was very clearly shown in the panels.
That's why I said Sungji did forget the ring and I think she would still forget it in many way.
Seju can't steal the ring, she didn't have energy for that. When Sungji washed for Seju, she put the ring beside the basin and didn't wear it back when she had finished.
And if Seju really had taken the ring. It would be like she had to waited Sungji for going somwhere else then took it, and when Sungji left she would have to put it back on the table then when she wake up she will see it there. If Sungji remember 'bout the ring why she don't noticed where she had taken it off.