All of my siblings have started developing for their age. My brother is currently showing interest in them. How would you approach this situation?

I have 7 siblings. 3 brothers and 4 sisters, and all of them are single. I have no idea how to approach them. My brothers have expressed interest in their siblings in my presence multiple times but they haven't done anything about it. This has been going on for years but now that they're all around 12, 15 and 18 I have no idea how to approach this. I don't want them to make a wrong move and end up getting arrested for God knows how many years. I'm just their older brother, so I don't want to appear as a dictator. Honestly, I don't even know where to start. If you have any advice, please comment.