Don't forbid your sisters from dating your friends
You recently made an excellent point that the only one who can decide who someone dates is themselves. However, I am a woman with many male friends I won't "allow" my sisters to date, and I would like to share that someone may have other reasons to step in. One is because my older sister dated a man I was friends with for five years before they met. When she broke up with him, he stopped talking to me. It was too painful for him because I reminded him of her.
The second is because although some are great to hang out with, I know more about these friends than my sisters do. They are NOT dating material. They treat me well, but their girlfriends, not so much. More of a maturity thing really.
So in a way it is protectiveness -- not just of my siblings, but of my relationships with my friends. Shouldn't I have a right to not want my family to date my friends so I don't get caught in their turmoil? -- Anonymous
Well -- wait. You have a right not to want it, sure. We all have every right not to want things. I have a right not to want people to use apostrophe-S to make something plural. That doesn't mean I get to edit peoples sign's, Christmas card's or mailboxe's.
(Okay, I did once secretly Sharpie out a calamitous apostrophe on a friend's knickknack. Sue me.)
Anyway. You have a right to want a certain outcome, but you don't have a right to make decisions for other people to bend an outcome your way. Not even when your intent is to protect others and certainly not when your intent is to hoard friends for yourself.
What you can do is respect the boundary between actors and audience. For your friends' and sisters' romantic lives, or for any romance you aren't personally in, you are strictly audience. You can offer warnings when needed and advice when asked, but otherwise take a seat.
Losing that friend over your sister's breakup was hard for you, no doubt. But treating people like your personal marionettes, not the autonomous equals they are, makes you the one controlling, patronizing and ultimately alienating people around you -- and the fallout from that hurts so much more in the end.