When they had kindled a fire (periapsantōn pur). Genitive absolute, first aorist active participle of periaptō, an old verb, but here only in the N.T. Kindle around, make a good fire that blazes all over. It was April and cool at night. The servants made the fire.
And had sat down together (kai sunkathisantōn). Genitive absolute again. Note sun- (together), all had taken seats around the fire.
Peter sat in the midst of them (ekathēto ho Petros mesos autōn). Imperfect tense, he was sitting, and note mesos, nominative predicate adjective with the genitive, like Joh 1:26, good Greek idiom.