If they invite you to sit for them but you're unavailable to do the sit, then you can always politely decline the invitation in your inbox.

When you open your conversation in your inbox with the pet parent who has invited you to sit for them, you will see an option above their invitation message where you can either accept or decline the invitation. You need to select the 'decline' button to let them know you're not able to do those dates offered. 

We would always recommend you send a courtesy decline message to the pet parent and let them know why you're not able to attend the sit, and maybe you could do any of their other dates in future!