I don't think you can force her to speak kurdish without damaging your relationship. Keep speaking Kurdish, maybe find something that she loves that only happens in the minority language (DS1 watched TV in the minority language long before I let him loose on Cebeebies!).
DH says most of his bilingual peers realised around that age they didn't need the other language to live in the UK and lost the skill. Keeping up the speech will at least allow her to have the understanding. I'm a monoglot.
I speak Kurdish. DH is 1/4 Kurdish, and speaks a bit, and knows quite a lot, albeit with a rather bad accent, since living with me. The DC have been raised bilingually, speaking Kurdish. My younger three (4 and 5) speak it to me, to relatives and sometimes between themselves, and are fluent. DD1 is 7, she's never spoken to or wanted to speak to another sibling in Kurdish (for example, when we said 'hello x' to DS as a baby, in Kurdish, she'd say it in English, we tried to go by example but didn't press the matter) and speaks it to relatives but to DH and I in English. I answer back in Kurdish and she understands fine. We are living in England, she has excellent English skills, and when she's with friends r whatever, I talk to her in English etc; but it's starting to get me down. If I have a conversation with them in Kurdish, she's the only one talking in English.
I know that ultimately, it is her own choice. I don't want to force her to learn Kurdish and I appreciate she probably seems very distant from my culture- we aren't religious and therefore miss out on quite a few things which would maybe have made her feel more connected. She knows Kurdish but it has started to get weaker noticeably, and I am keen that she can communicate with relatives and so on. Most of her friends are bilingual (she's the only Kurdish speaker, her friends are bilingual in Bengali, Punjabi, Hindi and Tamil mainly) so she isn't the odd one out in her peer group.
Any advice to encourage her to speak more in Kurdish, or at least retain the Kurdish she has? Is there any point?