My sister has leukemia how many times could i donate bone marrow not just for her but if someone else in my family needs some.?

Technically > once: Unlike other solid organs, bone marrow is a renewable resource as long as it is not damaged by chemotherapy or radiation. God forbid if you have to donate more than once to different or the same patient, allow some time (about 2 months for bone marrow and perhaps 1 month for peripheral blood stem cells) between your generous donations.

Theoretically yes.: Odds are that you will not match multiple members of your family. But you could give multiple times, to multiple family members theoretically.