Who is applying for the visa, your sister or your father?
If your father is the one who needs the visa and you or your sister is a citizen, that makes it more difficult, because then he would be an Immediate Relative of a USC and therefore highly suspected of planning to file for AOS after entering the US.
But whoever it is, they can apply for a B2 visa stating medical treatment as the reason. The nature and duration of the treatment has to be properly documented. If it is possible to do the compatibility testing without the foreign-based individual entering the US, make sure that is done before applying for the visa. How the treatment will be paid for also needs to be documented and shown to the consulate.