Why do you the that Estha and Rahel had an incestuous encounter?

This is a difficult question and really up to interpretation. I'll give you my take but there are no easy answers to this. Rahel and Estha had been away from each other for 23 years. Before that, they functioned as one person. They seemed to know each other's thoughts; each other's emotions: they shared the same spirit. After being ripped away from each other both were missing a part of themselves. There was so much tragedy and loss in their lives. They were further wounded with the death of their mother Ammu who died alone in a cheap hostel. When they finally see each other again there is a communion: two halves trying to become whole again. I don't think sex between the two can be defined in the same way as it might be for couples in love or looking for a sexual experience. They so desperately missed each other and this is how their pain expressed itself.