IMPO you have owned those kittens for much longer than they have... they wend on holiday and emailed you to state they were staying? with no definate return date..
my response is.. they are loved obviously by you... explain to them you have grown attached to them due to not knowing what their future held..
nothing in writing means no proof of thier ownership..
I was in a court battle with someone who was looking after 2 puppies for me (in the middle of moving).. a (past friend) cost me thousands and I finally got the two back months and months later.. but the police would not let me take them back.. even though I had bred them myself and could prove my ownership!!!
So comparing ... the other couple have no leg to stand on.. even if they sent police round....