How should I strengthen my floor joists that are too small?
Your joist timbers are very small dimension (top to bottom) for a length span of 3.6m. I suggest that when you have sistered on the few additional side beams that this is barely enough to help unless you at least doubled (or more) its top to bottom dimension. 

You would probably be better off adding additional support at right angles to these puny joists that spanned the 3.2m width of the room. You could do this at two places across the room to roughly break up the spacing to thirds across the length of the joists. The ends of these beams can rest on posts by the side walls. You could adjust the position as necessary so that any doors could still operate OK. The height of such beams would be a key factor at stiffening up the floor above. Obviously the higher the better but if you were able to make a beam for this with some a couple of 45mm thick members with plywood sandwiched in between it could be quite sturdy even if only 170mm in beam height.