While one may be sure that when Madhu is permitted to realize the actual situation, she will be entirely on RK's side, that may perhaps not be for a few more days.
Her focus was on RK's anguish, and she may choose to ignore what she does not want to realize as it would complicate things.
If Sultan were actually only involved on behalf of his mother and Madhu were correct in her belief about him being merely a friend and the father of a child she saved, then she can hope to have this mess sorted quickly, and then in her usual manner completely deprioritize the focus of her justice-crusade and refocus entirely on RK.
And on the other hand, if she has to realize and acknowledge that the man had such intentions for her, it would mean that RK's claims were correct and she was mistaken in her judgement of the other man's character and his perception of her.
It would make the matter more of a headache to get sorted, and would be a continued distraction in her and RK's relationship for a longer while rather than getting untangled and then being put as an adjustable extension into the existent dysfunctional family.
But her dialogue to Radha about stopping RK from killing Sultan to stop 'a brother from killing a brother' was beyond foolish, and would make sense only if it was to be placed in perspective against her later realization via the phonecall that there was no rightful justification for her to force RK to accept as brother someone he despises in manner very different from his tolerance for Sikki.
It would be like asking Padmini how she could have shot her husband Ballu, which would have been an equally foolish thing to say. The difference is that Madhu's characterization was intact then, and is not now.
Otherwise, she would have only been thinking now of keeping RK from killing someone, as she did when he was about to kill Ballu. There had not been any nonsensical dialogue to RK then about not killing her father or his father-in-law.
Whereas this time there is the 'brother killing brother' nonsense about someone who is as much a brother to RK as Ballu was father to Madhu. Neither relationship may consider a reconciliation other than repulsive.
And Madhu saying 'brothers' doesn't make it anymore acceptable a relationship than all the other gamut that has been tried to link that one character to Madhu and/or RK somehow.
Hopefully we may soon see an end to the epidemic of characterization-butchering in MEIEJ, and Madhu's reactions may stop reflecting poorly on her without even being true to her characterization.
Looking forward to the moment that Madhu stops being to made to uncharacteristically plead unrelateable stuff about 'brothers' and takes a stand against the other guy with the fierceness that is truly in her characterization.
That moment may be soon or later, but the serial will truly be worth watching from that point onward, with the reassurance that at least Shamsher Mallik won't be brought back simply to be the next character to get butchered. Poor Mohan Kundra wasn't spared the characterization-butchery even in the afterlife, but let Shamsher be spared in absence at least.