I'm here after watching the ending; I don't understand either!

I get that the FL was married to the former husband, but him watching their own sex tape? Concurrently as the ML & FL get married, and his sister who has a crush on ML finds out? Whut?

Ahhh that makes sense.

Was wondering what would happen next from the sister's expression, like she was angry/might scheme to use this against the FL. But then again, FL & her brother were legally married, so that video wouldn't have stood any case. Unless it was during the time the leads broke up, and they did the deed then. But maybe I think too much. LOL.

 ColourMePurple:

It just means that the ex-husband has hit his rock bottom where there is nothing more he can do now that the leads are getting married. So he is just watching his video with her while drinking himself to sleep. And the sister witnesses it. She has already given up on the TianYu. And it's not like TianYu doesn't know XinXing had had sex with her husband. So that video means nothing but to show the miserable state her ex is in. That's all.

Totally agree with all you said. Rock bottom for ex. His sister thankfully was much clever to see that there is no room for her in his life.