Meeting My Younger Half-Siblings... Help!

I was put up for adoption after my birth-mother got pregnant in high school. I'm 26 now, and found out we've been living in the same city since my birth, and last year we reconnected (through this site!). I've gotten to know her and her siblings, and her parents and the extended "adult" side of the family- and those relationships are continuing to grow which is great! The problem is though, everyone openly knows about me and has interacted with me EXCEPT for her four sons (ages 9-15), who she still hasn't told about me (I'm female btw). YIKES! In a few weeks, I'm meeting the boys, and am just completely terrified. Obviously it would have helped if they had known about me sooner, but I can't avoid that now and am not trying to play the blame game, it just is what it is. I am excited, but also, wow, they're 9-15, that's a volatile age group. There's no telling how they'll react. Has anyone ever had an experience like this?