What is a reasonable price to ask for second and third dogs in the same family?

I would look around at sitters in your area and price accordingly. I personally charge $10 for the 2nd pet and $5 for each pet after (for house sitting), and for drops I charge $5 for each additional pet.

We take off $10 for each additional pet.

I charge full price for each pet. We aren't making a lot of money for our services and extra animals mean extra work.

I give a $10 discount for each additional dog from the same family for boarding and house sitting.