keeping it as drawn now, imagine you move your Lav drain over, way over, by going through the wall studs. Come down into the joist bay where it will meet the new 1/8 bend of your new shower drain. This puts the (wet) vent closer. Also, the lav drain doesn't have to turn corners. This is what I meant when I said to imagine the best possible configuration without the joists. As it turns out, you only have to drill one new hole. Also, you get a clean run of joist to "sister" and strengthen. But, hey, what do I know, this might be all wrong. Wait for at least one other post confirming this. If you do this, you also have to figure out where to put a clean out. A shower niche or a shower bench will be a good means to hide the access panel to a clean out.