So, this is something that a lot of people have been thinking, I am one of them. Very basic actually.
Siya and Kabir being in it together
In my POV, Vansh and Riddhima are the same - the player as well as the pawn. For Vansh, all that matters to him is his family, especially Siya. For Riddhima, all that matters to her is Kabir.
In both scenarios, Siya is the family and Kabir is the lover - which are as a matter of fact Vansh and Riddhima's weaknesses, not strengths.
It is quite unlikely that Siya and Kabir are as straightforward as they have been projected. There are numerous instances which highlight that they are in this together:
1. On Kabir's plea, Riddhima agreed to be his spy or rather pawn to bring down the mighty VR. But on the cruise, she feels so uneasy because of Vansh's siblings. The only person to comfort her was Siya. She consoled her, gave her words of hope, encouragement.
2. A girl who has been wheelchair-bound for the longest time knows exactly how to handle her clothes. All of a sudden her dupatta gets stuck in the wheelchair. Riddhima knew that she was helping her, Vansh did not. Vansh does not trust anybody. So how did Siya trust Riddhima was coming to help her? It's like she was orchestrating a trust-building exercise for her brother to trust this girl. Same when she came during first aid to make her brother realize that he was wrong since Vansh only listens to her.
3. During the breakfast at the cruise also, Vansh was busy looking at Riddhima, and then she was building Riddhima's case in front of him. At his mansion, she was the one to stop Riddhima from leaving and requested her to accept the offer.
4. When Riddhima was interacting with Kabir over that make-up kit aka transmitter, he told her to end her stint and come out of Vansh's house, knowing all too well Riddhima is a woman of her word, and she will keep her end of the bargain because she loves him. The same goes for Vansh who hired Riddhima as a physiotherapist because Siya displayed this new grown fondness for a girl she just met once, which is just so strange.
So even though, the CVs are so ardently over-emphasizing on oh-how-dedicated-an-officer Kabir is or oh-how-sweet-little-sister Siya is, it's not believable sometimes. Thriller me saaf niyat ke log hazam nahi hote. Siya and Kabir could also be a couple, you never know. Vansh and Riddhima are quite clear in terms of characters - one is bad and the other is simple, which is bound to change later.