I thought Arpi was really stupid when she swore by the mangal-sutra. After all, when he doubts your fidelity towards your marriage, swearing by your marriage ain't exactly a bright idea. And her dragging out her confessions just made a bad situation worse.
And Avi was right - when Arpi first went w/ RV, she didn't think about Avi's anger then? Incidentally, if Arpi were to tell him that she was worried about him killing RV, which is true, that would only make her ugly situation worse.