The South End needs many restaurants that current commercial rents will obviously not support, like an Indian joint. With apps in the $8-14 range, and entrees running $20-30, Sibling Rivaly won't provide any relief to folks looking for a new mid-priced meal out.
It's a huge place that looks a lot smaller, thanks to being divided into several manageable-sized dining rooms, and has a small bar. It seems upmarket without feeling too stuffy.
The gimmick here is two chefs, brothers, cooking around a common set of seasonal ingredients. Usually one does an entree that includes the ingredient (e.g., beets), while the other does an app. The result is a pretty interesting, broad New American menu. Wine list seems to have some balance, not absurdly tilted to high-end wines, a few decent table wines for those of us not on expense accounts.
I tested a few things on a first visit: a cocktail, a glass of wine, an appetizer, a side dish. I was unimpressed with the mixology (they need to learn how to chill cocktail glasses), liked the appetizer (a couple of oversized raviolis in two interesting sauces with a great topping of shreds of short-rib-like beef, masquerading on the menu as pierogis), loved the side dish (nearly a mini-cassoulet: flageolets and slices of good sausage in an animal-fat-rich sauce and a good crumby topping, baked in a little ceramic casserole.)
The place is brand new; anyone who visits in the first few days deserves whatever they get. I'll reserve further judgement till I get a full meal here. But I'm guessing that's going to set me back upwards of $80-$100 inclusive with wine. At that price point, I'm going expect some solid service and great food. They've set up shop in a rather competitive neighborhood.