Hi C.,
My son did the same thing. He had just turned two right before our second was born. He started having terrible naps and coming into our room over night and it was stressing me out. I always stressed when he wouldn't get good naps or good overnight sleep b/c it, of course, affects their mood/behavior thereafter. He had the trouble napping and sleeping through the night again for about 4-5 weeks after his brother was born, but now is back to his old self, or even better, taking a longer nap everyday and sleeping about 45 minutes longer every night.
I think the excitement of the new sibling and the "trauma" of momma and daddy being gone for a few days probably makes them OVERtired, which in turn makes it more difficult to sleep well, which keeps the cycle rolling. I believe it was exhaustion from all the excitement, but also he was starting to work on his 2 yr. molers, and we had NO CLUE!! One day while brushing we saw two of them were starting to cut through the gums and we thought, "Wow - he never complained about it!" but are now wondering if he trouble sleeping was related to the teeth, since I hear teething can make them restless. One thing that I did for my own sake was to not stress about the sleeping. I don't know if that was part of the solution or not, but it certainly helped my state of mind. Maybe he sensed my calm? I don't know, but I decided that what he slept he slept and I couldn't do anything about it (b/c I had already tried EVERYTHING) so I would just need to relax and let be what was. Within 2 weeks of doing that, he was back to his old sleeping routine, and like I said earlier, even BETTER sleep.
I don't know if this will be the same for you, but I hope this was at least encouraging and helpful for you!
Best wishes with the two little ones!! It's a lot, but SUCH fun, huh?