Is it permissible for my parents to deal with sorcerers who recite the Quran and speak with jinn, and who asked him to slaughter a rooster and bury its blood, and to use herbs for incense to treat my sister? Is it permissible to pray behind them? And do sorcerers really pray?

The fact that the two men made such demands of your father indicates that they are charlatans and sorcerers. It is not permissible to seek their help for treatment, and one must be wary of them. If they perform polytheistic acts, such as invoking other than Allah, then it is not permissible to pray behind them. However, if they do not commit any act of polytheism, then prayer behind them is valid according to the preponderant opinion, because the preponderant opinion is the validity of prayer behind any Muslim. Sheikh Ibn Baz, may Allah have mercy on him, was asked about praying behind someone known to invoke other than Allah, and he replied: "Whoever prays behind someone who associates partners with Allah, his prayer is not valid, as long as he invokes other than Allah, or seeks help from other than Allah, or makes vows to other than Allah. One should not pray behind him, according to the people of knowledge." As for a sorcerer praying, that is not an issue, for Satan may command them to pray and perform other good deeds to deceive people and mislead them.