Is it permissible to give charity to a poor brother-in-law whose disabled daughter needs expensive treatment, knowing that the woman giving charity has enough money for her own needs?

It is permissible, rather commendable, for you to give charity to those mentioned, and you will receive a great reward for it. For Allah does not allow the reward of those who do good to be lost. Allah Almighty says: "Who is it that would loan Allah a goodly loan so He may multiply it for him many times over?" And His saying, the Almighty: "And whatever you spend of anything, He will compensate it; and He is the best of providers."