re: Advice on wall repair

Barb- since you have to do this repair anyway, you might want to consider a larger window. It’s pretty simple to frame in, although you would need to do the ‘outside work’ too. That said, what’s above that ceiling? Just the roof? Or another story? What’s the roofing material? You should have no problem replacing that crossmember, like for like. I don’t think it’s a header, but more of a nailer (which is good!). I don’t see any vertical supports, so I’m guessing those are just there to attach wallboard, not really for structural support. 

Given that it’s been standing on its own, there shouldn’t be an issue. As stated, you need take the OSB boards out on either side and expose the rest of that damaged board. You need to keep going until you get to no more rot/discoloration and then you want at least 24 inches of overlap for your sister, unless you use metal straps to support it. There are actual code sections that will tell you how many nails, what pattern, etc, but the reality is you just need to make sure that you have really solid overlap and nail it all together. If you go far enough sideways, you may find where the vertical members are . Then you could just replace the whole thing. Wall board is easy to put back up. FWIW, nails are better than screws since they’re stronger in shear, but either will work fine here. 16d nails if you go that route, IMO.