Really sorry to hear about that Squinty. Know the feeling though, we lost our Siamese (Taj) to poison also, it's an awful site to see before they put them too sleep!
We have another cat that sulked and moped around for a week or two after Taj passed away, lost a bit of weight also...not that that's a bad thing as she is spoilt fat rotten...lol. But after about 2 weeks she started coming good again.
We ended up getting another Siamese kitten about 2 months later, there was a bit of snarling, swiping and furry bits everywhere for a few weeks, but they are great mates now.
Your other cat will come good, just needs some time to adjust. Another cat probably won't hurt, as it would keep your cat occupied with who's boss within the household lol.
Wish you all the best.