Okay here's my theory: SungJi leaves Sumin for good, while Seju is waiting for Sumin in her mothers house. Sumin will try question SungJi, but it'll end up them walking separate ways. Sumin will try get SungJi back, but she won't allow it, because SungJi knows Sumin still loves Seju. Sumin will eventually give up and run back to Seju, but it'll be too late, as for Seju, possibility killing herself.. While Sumin is is crying in heartbreak. Leading her to try get back with SungJi once more, because Sumin would have no one. Just a theory tho.
Okay here's my theory: SungJi leaves Sumin for good, while Seju is waiting for Sumin in her mothers house. Sumin will try question SungJi, but it'll end up them walking separate ways. Sumin will try get SungJi back, but she won't allow it, because SungJi knows Sumin still loves Seju. Sumin will eventually give up and run back to Seju, but it'll be too late, as for Seju, possibility killing herself.. While Sumin is is crying in heartbreak. Leading her to try get back with SungJi once more, because Sumin would have no one. Just a theory tho.