Some random ideas:

-I was thinking Sceleritas would annoy AA by always reminding him of the only reason Bhaal is letting him live:
(for creating more heirs for Bhaal together with Dark Urge), and being pushy about it if it hasn't happened yet. hahaha Like a nagging mother-in-law who wants grandkids.
-Sceleritas scrubbing/waxing the floor conveniently there when he knows AA is about to wake up and walk out of the room, making Astarion fall prone. :p
-Hiding or smearing AA's hand mirrors or pouring blood over his vanity so he can't admire himself.
-Hanging up skeletons in random places where he knows it will jump-scare AA. xD
-Spraying AA's clothes with a sulfur scented perfume.
-Replacing AA's expensive wine with rat blood.

And Astarion always with this face when he's around:
[Linked Image from i.ibb.co]
biggrin

P.S. Your Sceleritas photobomb idea (photobombing their portraits over the years) was the best. Still makes me laugh!! xD