How did you know it was his?

Ick!

.... my guess is she told her

Yes, but the OP had to confirm that said penis belonged to her brother.

Where on earth did you get that idea? All we know is that the friend forwarded the picture. She probably sent it as a screenshot or with an accompanying text "look what your brother sent me". Almost anything is more plausible than that OP recognised her brother's junk by sight... O_o

Before you realized it was your brother’s, you know you enjoyed it.

Yikes

Normally the only people who enjoy those pics are the ones whose dick is on the pic.

I hope your brother is over 18 or else that is child pornography.

Not sure if she is a troll or an idiot, anyways that was a dick move from her.

The only thing worse than unsolicited dick pics is your brother's unsolicited dick pics.

The real question is were you impressed

Sweet home Alabama plays in the background

Looks like she didn't want to suffer alone.

people like that make me wonder about mankind

It’s often a tactic to embarrass the person. Especially if the harassment has been ongoing.

tl;dr: EEEEEWWWWWWWW!!!!!

I can see forwarding it to the mom or dad, since they were responsible for raising him so it could be partly their fault he's so creepy, and possibly they could influence him to change his ways.
But what does the poor sibling have to do with it!? I can't see how it's her fault at all, so why traumatize and mortify HER?
My gawd. If I ever saw a relative's dick pic it would destroy my sex drive for life. I'd need electric shock therapy to erase the memory. So horrible.
Possibly she'd deserve that if she gave him the friend's number, knowing bro was not a classy dude. But my bet is he just copied it from her phone without her permission, or had it for a legit reason like carpooling.
If OP really was innocent, the friend sending her the pic is arguably worse than the brother sending it to the friend. A relative's dick is so much worse than just a typical unwelcome dick.

I think the bigger problem here is that some guys still think their junk is attractive