So sorry to hear, yinyang - but glad he didn't suffer.
Yes, cats do grieve, in their own way. I had litter brothers many years ago, and the one died at the vets - his brother wandered around lost for several weeks. He missed his brother. Yang may well come round and turn into a lapcat - in our experience, one cat often overshadows the other, although it's not evident at the time. Brother 2 really came out of his shell after Brother 1 had passed away. Wierdly, he died exactly a year later, on the same weekend... it was not a good weekend for us, each year.
Good luck to you all.