This has been happening with greater frequency over time. My mother will send out a forwarded email (usually a joke, sometimes a large attachment of photos or PowerPoint). A day later, my father will send out the same forwarded email. This has gotten so problematic, my brother has flagged both of our parents' addresses to spam (which means I have to send him a note or call him if they have something important to say).
I have previously taken my father to task about the content of his forwarded email, explaining it is inappropriate to send jokes about octogenarian Scrabble to his daughter. However, now I need to find a polite way to let them both know their double-downing on email forwards is alienating behavior.
Stop doing the bolded/underlined. All you're doing there is allowing the situation to continue. If your brother misses something because he's blocked them, that's between him and them; by inserting yourself there you make yourself responsible for their communications which is always a bad idea.
If they are reasonable people then "Please stop with the duplicated forwards; it's filling my inbox and I may well miss important things in all the noise. I'd hate to miss out on Aunt Gertrude's eleventy first birthday because of that."
If they aren't reasonable, then there are no magic words that will get what you want without pain and anguish, so you have to decide which is more important, harmony or an uncluttered inbox.