Get proof and show her your proof. Telling is not the same as showing. Good Luck with this.
Tell her... gently.
WELL, MY FIRST THOUGHT WAS TO KILL HIM. BUT, YOU SHOULD AT LEAST TELL HER. THE SOONER THE BETTER. & BACK IT UP WITH PROOF. ... I WANT TO SHARE SOMETHING. YRS AGO I WAS TOLD SOMEONE HAD SLEPT OVER A GIRLFRIEND AT THE TIME, HOUSE. WHEN I WAS TOLD I DIDN'T BELIEVE IT. WHAT A FOOL I WAS. IF SHE'S DEEPLY IN LOVE WITH HIM, SHE MIGHT HAVE A HARD TIME BELIEVING IT. MORE LIKE, REFUSING TO BELIEVE IT COULD BE TRUE. .... THE SCUM BAG.
Go to HER with the information,.NOT him.
Since blood is thicker that you know... might want to let her in on your discovery. (With her seeing it as a fact, that ways her brother is not just messing with her.)