I be more worried your sister would accidently hurt a small breed pup being disabled. Remember small breed pup are incrediable fragile and can easily be killed or seriously injured.
What about a calmer small/medium breed adult? Small breeds are prone to snappiness, but in the shelter you'll find plenty of adults that aren't snappy and are okay with children with special needs
Honestly tolerant wise these are the most tolerant breeds
Golden Retriever
The Golden is a gentle, confident and loyal dog that is cheerful, trustworthy and responsive to training. This breed makes a wonderful family dog that is friendly with everyone – both human and other animals. This Retriever does very well with children but toddlers should always be supervised as young untrained dogs can be over exuberant. Goldens have a tendency to mouth everything and everybody so should be provided with lots of toys to carry around. A Golden remains goofy and puppy-like for many years so early obedience training is required. Goldens are eager to learn and can be trained to very high levels as seeing eye dogs, narcotics dogs and obedience competitors. It should be remembered that this Retriever is an outdoor hunting dog that needs two good exercise sessions each day. Goldens are suitable for first-time or novice dog owners
Labrador
There are not enough superlatives to describe this breed. Labrador Retrievers are one of the best family dogs in the world. The Lab is gentle, loyal, intelligent and extremely reliable with children. These Retrievers are not apartment dogs but can adapt well to city living if they are given sufficient exercise. These dogs love to swim and should be given lots of opportunities to retrieve sticks from the water in all kinds of weather. The Labrador is good natured and eager to learn and can be easily trained to do everything from competitive obedience and field trials to becoming a “seeing-eye” dog for the blind. Most Labs love to chew and they should be provided with lots of chew-toys that they can carry around in their mouths. Some male dogs can be stubborn and try to dominate the owner. Early obedience training will make this breed much more manageable. Despite welcoming most burglars into your house, the Lab will let you know they are breaking in. Labs make pretty good watchdogs and do well with first-time dog owners.
Scotch Collie-Oddly enough in australia, they are used as apartment dogs!
Newfoundland
The Newfoundland has a wonderfully sweet and gentle disposition that is reflected in his kind expression. This gentle and good-natured giant dog is great with children and makes a terrific family dog. Toddlers should be supervised carefully as one slurp from his big tongue could knock a little one over. Newfs are quite intelligent and can be trained fairly easily using praise and food rewards. Newfs and all giant breeds should be socialized and obedience trained early while puppies and through adolescence. All giant breeds must be well trained. Male Newfoundlands can be a little more independent than females. Newfs need lots of companionship and need to be involved in family activities. Some Newfs are shy but generally get along very well with strangers. Newfoundlands are suitable for novice or first-time owners.
The Newfoundland is slow moving and has a tendency to be lazy. They enjoy and benefit from play sessions, daily walks, and swimming. They are relatively inactive indoors. The Newfie will do okay in an apartment or condominium dwelling provided they are given sufficient exercise. They enjoy a run in a small fenced yard.
We have a newfoundland near me-however quite abit smaller then normal newfoundlands, this one only slightly bigger then my Labrador x Golden Retriever and she big for her mix!. Gentle as anything, and gets alone well with other dogs, and don't think you'll have a problem with it with cats
larger is better and these are great with Children, they are more patient tolerant breeds and most will not snap or show any sign of aggression, even when accidently trot on there paws, pull there ears, pull there tails etc, and are gentle once obedience trainned.
Remember all puppies nip because there teething. Are you aware some larger breeds can actually do well in a small yard? Newfoundland and Scotch Collie are the two that are good!
1.What You See Is What You Get. How big will the dog get? What will its temperament be? Is it easily trained? What will its personality be like as an adult? Will it be hyperactive? Adult dogs are, to steal a term from Internet lingo, WYSIWYG (What you see is what you get.) All of your questions are easily answered, because the dog is already an adult. You can pick large or small; active or couch potato; goofy or brilliant; sassy or sweet. Further, the rescuer and/or foster homes can help guide you in choosing just the right match for you. (Rescues are FULL of puppies who became the wrong match as they got older!)
2. Unscarred Children (and Adults). If a puppy does not teeth on your possesions, it will teeth on you and your children. Rescuers often get calls from panicked parents sure their dog is about to seriously injure their children. It usually turns out the puppy is just doing what puppies do, i.e., mouth or nip. Parents, too emotional to see the difference, just want to get rid of the dog. A growing puppy is going to put anything and everything in their mouth. It must be taught bite inhibition. As the puppy grows, the puppy's jaws become stronger and its teeth are replaced by its adult teeth. The mouthing and nipping it did as a puppy now can have serious consequences. Far better to get an adult dog 2years+ that has "been there, done that, moved on
Also puppies shouldn't be left along anymore then 2hours.
Remember most small breeds won't tolerate a tail pulled, or ears pulled or fur pulled
Some suggestions
American Eskimo dog=6-40pounds
Bred solely to be companions, the American Eskimo Dog thrives on being part of the family. This charming and playful breed is excellent with children, and loves to be lavished with attention. They are suspicious and wary of strangers and make good guardians of their family and home. They love to bark and will sound the alarm whenever they deem it necessary. This breed does not do well if left alone for extended periods of time or if they are isolated, as they will engage in destructive activities.
Beagle
http://home.earthlink.net/~happyacreknl/id9.html
http://www.threeseven.ca/2008/02/truth-about-beagles.html
Bedlington Terrier
Bichon Frise
Bolognese
Border Terrier
Boston Terrier
Cairn Terrier
Cavalier King Charles Spaniel
Cesky Terrier
American Cocker Spaniel-Can be snappy with small children
Coton De Tulear
Dandie Dinmont Terrier
English Cocker Spaniel-Can be snappy with small children
Glen of Imaal Terrier
Havanese