Jim and Jay gave you good advice. If this is the section around the toilet or a section next to the toilet or sink that won't get walked on, then a solidly sistered patch to hold the weight of the toilet and your 400 # brother in law should do it. If it is to be walked on regularly, then spanning the joists may be your better bet but it depends upon your flooring materials. If plywood, then go ahead and span it for sure. If hardwood flooring then a patch sistered or on the joist centerline may be better. Remember that the thickness of dimensional lumber, flooring, and plywood has been shaved off some a few years back so you may need some layers of roofing felt or shims to get it level.(1/2" plywood is now about 7/16" and 3/4" is less also) Use galvanized screws and not nails for a better fix.