I think they're trying to tell Kaito that they don't need him as a male vocaloid anymore because Len is better. That's Kaito's grave in the last panel if I'm not mistaken.
Very late, but think the joke is that it's implying they're prostituting themselves to earn money. So when Meiko says it...
Even Miku...!Rin too!?Ah I knew it.I had thought so!To be honest, I also...Nii-chan...
I have been keeping it a secret until now, but the truth is that Rin has been doing the same work as Len...I also thought that if I could support the family even a little...