There can b two reasons why Sahir had a sudden change of mind or heart in this case ...
First reason could b he felt guilty of backstabbing Aarzoo after seeing the honesty and purety with which she said yes for marriage and then hugs him ... In that last scene , I could see sheer guilt on Sahir's face when he holds her back and she clutches onto his back tightly ... I guess this is where his game ended the moment she hugged him with a yes for marriage .. Mayb he thought , thats it , I cannot go ahead with this game anymore by keeping her in dark , to hell with the deal and Rehman , I just cannot destroy a girl's life by taking her for a ride especially when I hv myself gone through similar betrayal in my past 🤔 .. This thought might hv resulted in the real Sahir finally waking up and putting an end to this love game by telling her the truth about his marital status 👍🏼

Second reason could b Zaki's words to Sahir in the beginning of the episode might hv resulted in Sahir finally deciding to put an end to this whole love game and before Zaki tells her the truth , he himself wanted to tell her the truth and prove to Zaki that he is not backstabbing or cheating Aarzoo from behind , the accusation that Zaki had put on Sahir about how he is backstabbing Aarzoo and then putting blame on his younger brother .. Mayb he wanted to prove to Zaki here that his love for Aarzoo is not fake , reason why he tells the truth to Aarzoo before Zaki could tell her anything .. Obviously its a last minute decision for Sahir and it came from his heart because his love for Aarzoo was real 😳 .. Sahir knew that Zaki wont stop this time even if Alvira tires to stop him , no wonder he himself went on to tell the truth before someone else says it 👍🏼

It could also be a combination of above two reasons since Zaki's words were already haunting him about how he is playing with Aarzoo's feelings and knows nothing about love , on top of that Aarzoo's honest confession in the end and the way she said yes for marriage with all smiles might hv further made him guilty and so finally he decides to put an end to this game of lie by telling her the truth
In precap Sahir also tells Aarzoo that He played this love game with her just how she too played with him which I felt was a deliberate lie on Sahir's part just to shove her away from him so that she does not further ask him about his marriage details when Sahir clearly knows that she never played any games with him 🤔 .. I think Sahir here is disgusted with his own self for playing with her feelings and so now he is deliberately presenting the bitter truth of his life in front of her in such a way that she instantly starts hating him 😭😭 ... But going by spoilers it seems Sahir will fall flat on his face once again when Aarzoo will land in trouble with the cab driver and he will go on to save her 😉😆😳
Sahir will not really achieve anything by telling her this truth right now since his main motive was to marry Aarzoo by hook or by crook so that Zaki accepts defeat and marries Sam which will result in the deal finally being done , reason why he desperately wanted to hide his marriage truth from Aarzoo since he knew well that if she finds out this truth , then no way she will marry him but even then he goes onto tell this truth to Aarzoo tomorrow which is a clear indication that finally his heart took over his mind and so now he no more cares for the deal but just wants to b truthful to the person he has fallen in love with even if it means getting hate in return from the same person 👍🏼