My hubby's sister is pressuring him to donate his kidney but I disagree. Whose call?

Donor. No one should pressure anyone to donate. In fact when the prospective donors and recipients come for an evaluation, they meet with separate teams to avoid any potential conflict of interest that can arise. The information on both sides is kept separate. Although it has been shown to be medically safe to donate, it is an altruistic and person decision and should not be forced.
His call. A donor has to be 100% comfortable with his decision and under no pressure. She can state her opinion and you can state yours. He should be the one to decide under no pressure of any party. If he has any illness like hypertension or diabetes he may not be a candidate after all.