Went out for dinner in what context? Friends? Date? Beginning of a relationship?
Obv. I don't know from the same POV as you, and I've not had my brother go out to dinner with any of my ex's, but I'll tell you one story that's a similar, though not quite so serious, predicament;
While we were in our first year of 6th form college, me and my life-long best mate (without telling each other through fear of exactly the same thing) basically traded girlfriends!
I took her out to help her get over their break-up and we ended up together, and he did the same with my ex too! (we got together around the same time, broke up with them around the same time, got together with the others' ex around the same time, and to top it all the 2 ladies were very good friends too!)
When it all came out a few weeks later, well, a messy situation doesn;t describe it. All 4 of us didn't know what to say, or where to look.
After a long discussion, we basically decided that, what they hey, if we all were happy with the new relationships, why ruin it just because of who we used to date? It didn't last long though, I think that the girls just found it too weird after a few more weeks, and called it off.
Me and my mate were fine with it all though, in fact I truely believe it brought us closer as we knew that we could even go through something like that and still remain best of friends with no hard feelings, even feeling happy for the other; and we are still best mates today (in January 2009 it'll be 18 years since we met!
)
OK, I waffled a bit there, and most of that may or may not have any relevance to the way you're feeling, but I guess the main questions that come up are;
1) why did you break up, is he a bad guy who might hurt your sister
and
2) if he isn't a bad guy and you just grew apart, is it right to not let your sister have a chance to be happy?
Obviously if he IS a bad guy and you split up for a very good reason (i.e. abusive) then you have a right to tell her so and warn her, but if the break up wasn't based on anything like that and the relationship just naturally reached its end, you don't want to come off as the jealous/scorned/hurting sister by trying to get your sis to back off.
It's a difficult situation, thats for sure, and it all depends on your relationships with each of them ultimately.
Sorry if that was a load of Bull, it's just my view, it might be skewed, I am a man after all