Little children repulse me, my brother tried sticking his filthy fingers in my mouth, I threatened him next time he does it, he will lose a finger... I hate the little brats, and will get a vasectomy in the future.
They're a bit like marmite.
(Not the fingers, children in general)
Not to everyones' taste
Your brother may only need reminding of some personal boundaries?
They have to learn that stuff.
They're not born with that understanding already downloaded.
It's an interactive, hands-on kind of learning.
Moving mouths can be fascinating to the very young.
Granted, your brother will learn very quickly the first (and last) time he ever puts his fingers near your mouth
There are alternative methods,
Keep your mouth out of reach?
A firm "No"?
redirection of brother's interest?
I absolutely get it.
Nobody sticks their fingers in my mouth; other than my dentist, and he's wearing gloves I've just watched him take from the box they're packaged in.
The transference of germs via hands and fingers is phenomenal.
Your immune system is equally as phenomenal but not the case in point here.
Make biting your brothers fingers the very last thing you do.
It's messy and will bring about a lifetimes worth of hassle. The consequences could be pretty dire
Try other methods to help him learn, first.
If you're stuck for ideas, message me