All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If your brother-in-law does not have means to marry and he is poor, then you can give him Zakaah but advise him to give up smoking and give up dissipating money in useless lawful things. Advise him also not to use the Zakaah money in forbidden things like smoking.
If you feel that he accepts your advice, then there is no harm to give it to him.
On the other hand, if you know that he will spend the Zakaah money in acquiring prohibited things or in squandering, then it is unlawful for you to give him Zakaah.
Allah knows best.