Hmm...you don't want to give the talk? So don't give the talk. It's not that hard to say no. Practice that a couple times. And you don't have to give a reason, either. But to be polite, if that is what floats your boat, you could say you can't for personal reasons and leave it at that. Kind of like this:
You: Brother, I can't give that talk Thursday night. I'm letting you know now so you can reschedule me or find a replacement, whatever you think is best.
Brother: Ok, but why can't you give the talk?
You: Something came up, something personal.
Brother: Oh? Can you tell me about it?
You: Sorry, Brother, it's personal.
Brother: Very well.
Or you could resort to a more passive stance. Just don't attend that meeting. Don't call the school coordinator, don't show up. Easy.
Or if you feel comfortable lying to an organization that lies to you, just make up an excuse.
Or if you wish for a more long term solution, just resign your enrollment in the completely "voluntary" school (yeah...just like blood fractions are completely up to your conscience...whatever).
Hmm...that give me an idea. If you do decide to give the talk, focus on the blood fractions and how they are a matter of conscience even if the book study overseer tries to tell you otherwise just so he can get done with helping his group fill out those annoying DPA "No Blood" forms.