I have a brother and sister pair that came from the same litter (7 months old) and although they are close to each other, they are both very bonded with me and the rest of my family. I have never noticed any problems with having the two of them together. I don't separate them at home- I do take turns taking them to special places outside of the house, but that is because we have a total of 4 dogs so everyone deserves a little special one-on-one time.
When we had Leo neutered, Mia was lost without him. It took her a couple of hours to realize he was missing
, but then I did notice her looking for him and then sulking several times over the course of two days.