I don't think that Seher will marry Ahil.
She responded to him via a DVD recording.
When they were with the rest of the family she looked at Rehan. She recalled how Rehan proposed to her and looked at Rehan when accepting Ahil's proposal. Seher felt very uncomfortable when Ahil went to embrace her.
She avoids Ahil whenever he wants to get close to her. If she was a vamp then sharing a physical relationship with Ahil will mean nothing to her.
Seher is feeling rejected at the moment. I don't think she will be able to marry Ahil in front of Rehan.
When Razia tried to convince her she refused. She said she understands what marriage means and cannot marry Ahil. She only agreed after Rehan said he wants to forget her and move on. If she heard Rehan mention that it's very hard for him to forget her she would never agree to the marriage. Razia would not be able to convince her with any amount of money.