I am attempting to convert my attic into living space. 

Originally, there were 2×8 joists up there, which I sistered with some 2×10’s for a 15′ span.

So far so good

I want to install a dormer up there, and in order to do that I have to extend the existing floor about 32″ in order to get the floor under the roof pitch far enough that the dormer window isnt sitting obnoxiously high. 

The section I need to extend out over is 2×6 framing (over the master bedroom) but those 2×6’s do not appear to sit on the sill plate of the outer wall, but rather are fastened to the roof rafters (the master bedroom has a beveled ceiling. So, rather than try and untangle what is actually going on there, and since I only need less than 3 feet, I thought I would extend the 2x10s that I just put in. I only need to do this with 2 joists. 

My Question:

What is the best course of action for extending these? 

My initial thought is to get an 8′ 2×10 and just sister it against what Ive all ready got up there (giving my a triple joist across the sill plate and about a 6′ back span, plus an additional 9′ from the original joists) 

Is my thinking all wonky here? 

Open to all suggestions here.