from BTS we got to know that zoya and haider are bro sis and i guess he will be a twin of zoya as both are similar in many ways and might be he was given in adoption to someone or may be lost somewhere otherwise zoya would have remembered him atleast once and when zainab comes to india she would have brought haider along with her too and gaffur would know him and would have waited for his son just like he is waiting for zoya and moreover gaffur has no heir and has only daughters..
so i feel
zoya and haider are twins and may be some incidents might have seperated them..
coming to haider and humeira...
if haider came to take revenge from razia he will know everything about them. and he is good enough to know that humeira has no mistake in all this and she will be his sis too.. he might have that sis feeling and hence he might be trying to get a smile on her face. he tries to make her smile but is not trying to flirt with her nor seems to fall for her as he himself said that ayaan will be back as no one can leave such a beautiful life and go away..
and the other theory is may be haider was close to zoya's mom and might have witnessed the gudiya factory incident and hence decided to take revenge on razia.
so he feels zoya as sis as he is close to her ammi and considers zoya's ammi as his mom too and in this way he can be a bro to zoya and fall for humzie too..
but however i want haider to be zoya's twin as i want baby to have some one of her own blood relation and i want ayra and asya no other pairs...