What are the current observational constraints on the existence of Nemesis?
Few important points about WISE:


it was able to detect anything with a temperature above 70-100 K, whereas the coolest brown dwarfs are in the 500-600 K range (the coolest was discovered by WISE itself, see Mainzner et al., 2011);
it was able to detect objects larger than 1km up to 3 AU from the Sun, or objects of 2-3 Jupiter masses in a distance up to 7-10 light-years (see here and here);
the closest brown dwarf detected by WISE is a brown dwarfs binary system, located at 6.6 light-years from the Sun.


So I would say we can be pretty confident that Nemesis existence has been ruled out.