If you don't intend to upgrade to full frame within the foreseeable future then I'd recommend getting the Canon 17-55 f2.8 IS or the Sigma 18-35 f1.8 as your workhorse lens for weddings and events. From my experience, Canon lenses tend to have a better resale value in the long run so if you do plan to transition to full frame you'll likely get more from the resale value compared to the Sigma.
The Canon 70-200mm f2.8 IS v2 is a fantastic lens, but it's very heavy and I rarely use it on my 60D because of the crop factor when I shoot indoor events or weddings. Instead, I use it on my 5Dmk3. However, if you're shooting sports then the crop factor could be a benefit for the added reach.
IMHO, Sigma 18-35/1.8ART is the best choice for crop at the moment. Without any BUT's or alternatives
One of biggest problems with the 7D is that there are no good L zoom lenses for general purpose shooting. There is nothing equivalent to a 24-105L or 24-70L with high build quality and weather sealing.
Technically true. There isn't an L-glass equivalent to a 24-105L or 24-70L for crop bodies. But, the 17-55 f2.8 is essentially L-glass without the weather sealing. True, the build quality of the 24-70 and the 24-105 feels better, but I wouldn't hesitate to recommend the 17-55 f2.8. Put a good clear filter and you'll "seal" it from dust.
Well, essentially, 24-105L costs as much as 17-55/2.8, but on FF it is better in every way + 7D2 costs near as much as 6D (which, IMHO, is much better than APS-C), so, for event photography 6D + 24-105L wins, essentially
. EF 24-70/2.8L'II is much more expensive than Sigma 18-35/1.8ART and its range is pretty awkward on APS-C, but on FF it is wider, longer and better (IQ, build, L) than Sigma 18-35/1.8ART on crop, so it may be worth it.
I say go FF (6D or 5D3) with the best lens you can afford