@Theresa-Quynh-Tho0 Then why did the siblings book your space instead of a space that could provide them a bed each?! Did they intentionally arrange to use your sofa as a bed?
Judging by my belief that the stain was made by your recent guest, you have clearly been victimised here and it should'nt be your responsibility to repair the damage. I feel that the difficulty might be in proving the same against the claims of your guest and this should be raised through AirBnb customer services initially and then maybe issuing a payment request to the guest for cleaning?
This, and many other guest transgressions against House Rules and damage makes hosting such a risk. Having a rule "No sleeping on sofa's" might be nit-picking and that wouldn't have prevented the stain happenning - only it would have happenned somewhere else instead!
I'm considering more an arrival contract itemising House Rules and expectations of behaviour as well as penalties for your such scenario. This might involve a car hire type contract with damage illustrated or listed before acceptance and keys being provided. Car hire companies manage this quite well but they retain either a credit card or hefty deposit, I've not explored this type of arrangement fully, but I wonder in light of your experience whether any other host has such an arrangement like this in place they could offer advice on?