Yeesh. If both humans are sick and in some pain, why did you adopt a
pair of boisterous kittens? Shelters are filled with older, mellow cats
who need quiet homes. Oh well. I'm glad you love them. What you have to
do is reinforce the behaviors that you want and discourage the ones
that you don't. When you see them up there - remove them - and give
them a brief timeout (10 minutes) in the bathroom. Do it consistently
and eventually they will associate being restricted with messing with
the window. Don't scold them, just gently place them in there. If you
see them heading there, say firmly (don't shout) - NO - and if they
listen give them lots of treats and if they don't - off to the
bathroom. If you are rigorous about it, they will eventually get the
message. Then repair the windows :

Good luck!