Sad that potential owner of Rollo pulled out ,they must have known the risk ,at least you did hearing test unlike a few breeders .
Glad you have a forever home for him.My Cocker Kira who died last year was deaf for 4 years and other sences took over ,he will be ok.Tell your sis to find a dog trainer experianced in training deaf dogs and owners.
Hugs and kisses to you and your puppies.