Yes you should.
They can tell your brother they found out themselves, there is no need to bring you into this discussion.
Online porn is not a good way to learn about sex, it might teach you where to put what, and more, but it does not teach anyone about the importance of a loving relationship, and without that lesson then sex is just sex for it's own sake.
I am a first or maybe second generation Dad where sex online is a big problem for our children (ie you and your brother).
In my day we could see pictures of the sex act, but with no movement because it was in magazines, the words etc we heard about sex were from our parents, the way they talked to each other in front of us proved more about how a loving sexual relationship should be than any other medium could.
An orgy is possible, but it is not normal day to day life, 3 girls and one guy is possible, but again it is not normal life, 6 girls in a pool playing with each other is possible, but far from normal life.
At a young age, ie 12, if you see this stuff, you can think it is normal and try to act on it, and that will hurt people because it simply is not normal behaviour.
I know you can put 6 guys and a girl in a room and everyone will have a good time, but it has to be by choice, and trust me, 99.9% of females are not that girl.
Just tell your Mum and Dad, or just Mum, or just Dad, but stop it now before anyone gets too confused about what is normal and expectable, and what is rape.
I'm red blooded, and anything goes, but you need direction to be taught to you before you go out and fcuk up the world.