Cousin is having an affair with my brother

Problem/Goal: My cousin (18f) is having an affair with my brother (20m).

Context: Umuwi ung pamilya ng pinsan from abroad and in 11 days that she spent here she managed to hook up with my brother.

At first, they were being chatty with each other which surprised my parents and her parents kasi for the past 6 months my brother had been suffering from depression ever since namatay ung gf niya from an accident and even tho he went to therapy multiple times sobrang gloomy pa rin nya at huminto sya sa pagaaral so nagulat talaga kami lahat kasi parang biglang nagkagana sya nung pagdating ng pinsan ko so even though we thought it was a little strange we were happy that he was acting like this for the first time in a while.

Pero over a week in, namasyal ung mga parents namin ng cousin ko leaving us alone with my other siblings and my cousins. Nagluluto kami ng hapunan tapos pinuntahan ko ung kwarto ng brother ko upstairs kasi kakain na tapos nakita ko na nakabukas ung kwarto niya and I saw them cuddling then bigla silang tumayo kasi narinig nila ung paghakbang ko sa hagdanan. Dun biglang tumaas ung suspetsa ko sa kanila and the next day pumunta sila sa hotel where my tita works at along with my other cousins in which hindi ako nakasama kasi na busy ako sa part time job at school works ko.

So the day after they went to the hotel tiningnan ko ung cp ng brother ko while naliligo sya and yun I saw their chat where I found out that they did hook up on the hotel. I also found out na may boyfriend ung pinsan ko kasi minention nya sya and my brother replied "dont worry, I'll make you forget about him while you're still here" and sinabi pa ng pinsan ko na this isn't the first time that she hooked up with someone while in a relationship and ewan nandidiri lng talaga ako sa kanilang dalawa. I took a video while scrolling through their chat and another thing that I found out was that they've been chatting since October of last year.

Previous attempts: I confronted my brother yesterday at sinabi ko na nakita ko ung ginagawa nila the day I saw them cuddling and I was checking to see if he would either defend himself or back out but he doubled down instead and said na wala akong ebidensya na may relasyon sila then he left (hindi nya alam na may ebidensya ako I just didn't tell him about it to see if aawat ba out of conscience pero hindi eh). I also showed their chat to my ate pero she told me to not say anything yet pero hindi sya makabigay ng alternative so naguguluhan ako kung ano ang gagawin ko apart from two options.

As I'm typing this nasa airport na ung cousin ko with her family and my brother is there hindi na lang ako sumama kasi gumawa ako ng excuse na busy ako sa trabaho at sa school works ko. The one thing I'm very conflicted is whether I should confront him again but this time show him the evidence and threaten to expose him if he doesn't stop or expose him to my family which will end up spreading to my other relatives which could possibly cause a lot of conflict with my cousin's parents and mine (my cousin's parents are the type to defend her actions because I've seen them do this before on a separate issue). Asking on which one I should choose or if there are any alternatives that you guys can think off plz thanks.