Good day TS!
As much as you care and as much you are affected with your cousin's current situation, it is not for you to intervene. You will be the one who will take all the blame if ever the impending "Wild Fire" spreads. After reading your post, I am very sorry to for the girl
(the one that your cousin duped). But I'm sure that in due time that scandal will be exposed to THE GIRL if it still continue the way it is. You seem to be a nice girl so please, for
your sake don't intervene because that issue is too personal for the three of them. It is a Love Triangle of which you are not involved. Ikaw ra gihapon makontrahan ana sis. The best thing you can do is to pray for them that they will realize their mistakes.