I'm pretty sure that Singer and Vaughn stated that Alec Summers is indeed Scott's brother...just simply older, which kind of isn't a big deal. The Rachel Grey thing is going to be a hard thing to explain away, unless they just make her Hope or something.
Really? That's a bit hard to believe. Think about it for a second, Alec is in his late teen in the early 60's, that means he would have to have been born in the early to mid-40's. Scott on the other hand would have been in his early 30's in "the near future" which let's assume is about a decade after the first film came out (now-ish?) so Scott could have been born around the late 70/early 80's. That's a nearly *40 year* age gap! Not impossible, but it strains believability somewhat.