This is a very interesting question to consider, because both Abby and Martha on the one hand are not given some of the most hilarious lines of the play, which are of course given to Mortimer Brewster as he tries to cope with the situation that unfolds about him. However, I think that the innocence and the very straightforward way in which these two spinsters talk about their "hobby" of murdering men and how this is presented is itself incredibly hilarious. Consider, for example, the following exchange:
Mortimer: Teddy's killed a man, darlings!
Martha: Oh, nonsense!
Mortimer: But there's a body in the window seat.
Abby: Yes, dear, we know.
Mortimer: You know?
Martha: Of course.
Abby: Yes, but it has nothing to do with Teddy.
Abby: Now, Mortimer, you just forget about it. Forget you ever saw the gentleman.
Abby: We never dreamed you'd peek.
Although Abby and Martha may not have a sense of humour in the same way that Mortimer does, it is their lack of humour and their no-fuss way of talking about what they get up to that is incredibly hilarious. As befitting their status as elderly spinsters who often deplore the way that society is going, they seem to be rather humourless, but that is something that is skilfully used by the playwrite to present them as absolutely hilarious characters.