I was worried about the exact same thing with the boys, when Riley had to be seen for his panting at around 4 months old I took them both for the initial appointment as I was so worried how 1) Riley would cope alone at the vets and 2) How Lukey would feel home alone abandoned by his Mum and brother
I had to take Riley back for the ultrasound on his own and they were both absolutely fine, I really was worrying about nothing. Was home for a bit with Lu but he was completely fine about it, a bit more meowy than normal but certainly not upset by his brothers absence at all and they really do love each other
Was also worried that Riley would smell of the vets and that Lukey would reject him but needn't have worried about that either.
I'm sure they'll be absolutely fine, it's only for half a day and the neutering is a simple op with very few risks