Mrinalini doesn't want Vishaka to get pregnant and have the WAARIS, as it would mean most of the property will go him/she. She & Vivek doesn't exactly have a relationship, hence no WAARIS on their side.
So if she learns that Radhika who she suspects is the love interest of Dev, is the Choti Bahu, she will actually try to separate them. She will take her chance with Vishaka. She already suspects they are not living as husband and wife. Remember the morning pooja after the shaadi etc. SO NO RADEV = NO WAARIS = MRINALINI ALONE, BITTER BUT RICH!
😆 RADEV r in a real pickle.