2 things are telling to me. The mom is on the sister's side, and when the wife went to talk to the sister, she apparently behaved in such a way that police were summoned, and charges were a possiblity.
I'm thinking the mother knows both her kids and sides with one over the other. Something pretty serious would have to be going on within the family for me to encourage my daughter not to donate bone marrow to my son. Now no-one is saying, but this is a mother about to lose her son and she's telling her DD not to donate? Simplest explanation is usually the truth, and in this case, the simplest explanation is that leukemia patient has done something heinous.
The wife's behavior here is pretty unusual too. Yes, I'd be desperate were I in her shoes, but I would also be able to remember, being angry, or assaulting my SIL is not going to make her donate. No need to make any prior issues worse. Beg and plead, grovel as necessary, but make such a disturbance that police get involved? Erhm, no. Not a good plan.
I'm not saying I agree with not donating. I would donate, but in such a way that the donee would not be around me, and make it clear this does not mean I resume contact with such person. And I admit freely, such donation would be done for my own peace of mind, only. Don't wanna try living knowing I could have helped and didn't.