I get the impression based on what OP said that none of those were problems. OP was involved in some way with GFs transition in diabetes and managing insulin, and presumably wouldn't have been against gifting her insulin. This indicated they know there is compatibility there. OP also seemed to indicate the medication came from an extra supply that didn't really affect him. OP also indicated that the brother is involved in his own insulin management regime. Put this all together and we get the impression that none of those are the problem.

With none of this being the problem, this just becomes the morale crusade. And I'm a little more forgiving to my family when it comes to victimizing them with morale crusades. Especially when it comes to helping someone we would all supposedly care about, that OP said himself he would have helped.