I dunno BB I'm deffo in a minority here, but if the d-i-l suffers from Lymes disease, then she may very well faint from over-exertion. You might not believe that she's sick, but it doesn't mean that she isn't sick - and working - unlike the son in law.

IF you both chose to let them off paying you anything so they could save, then you chose that. And they've saved - or so it would seem.

Three months Adult step-kid rebound isn't worth freaking out over I reckon. Sure it's changed things a bit for a little while, but hey ho, that's the joy of family!