Is it permissible to count the cost of medications paid for a bedridden sister as part of the obligatory zakat money due from the questioner, knowing that he supports her and her brothers contribute to her expenses to a limited extent, and he gives zakat to his poor married sister from the same zakat money?

The maintenance of a needy relative who is an heir is obligatory upon their capable relative. Therefore, if you inherit from your sister, her maintenance is obligatory upon you and your brothers and sisters, and it is not permissible to give her Zakat in that case. If her maintenance is not obligatory, then it is not permissible to count what you previously spent as Zakat. In the future, it is permissible to give her Zakat with the intention of transferring ownership, and there is no objection to spending it on medication for her treatment. It is also permissible to give Zakat to your married sister and her children if her husband is poor and unable to provide for them, and this is better than giving it to others.