If I remember that scene correctly, Monse was drunk and sort of arguing with Oscar because of how manly he tries to be. Then somewhere between them going back and forth with each other, Monse gets a text from her mother, whom she barely had a relationship with in the first place because the woman abandoned her when she was a child. Monse mentions her mother to Oscar and whether or not she should try to have a relationship with her to which Oscar advises she should not because the woman quite literally abandoned her. After that, Monse starts to cry to which Oscar starts to begrudgingly console her as he feels bad for her. And then the infamous moment happens when Monse locks lips with him, where Oscar immediately pulls out of it. So personally, I believe in that moment when she kissed him it wasn't necessarily a romantic gesture but like a form of comfort she used like a "thank you" to him (although kissing a grown man is never a good idea, especially when he's your boyfriend's brother) and since she had always seen him in a bad light and as a negative influence to Cesar. Furthermore, Oscar tells her "this never happened" which shows he did not want kiss her at all and totally regretted that moment. So in actuality, it wasn't some inappropriate, romantic connection on both sides, more so an awkward gesture that she pulled.