My mom just plain doesn't want a puppy.

The problem is, Shelties, like all sheepdog breeds, need a LOT of exercise. They are natural-born herders and are one of the few kinds of dogs that actually like to work. If they are not stimulated enough by being able to run around outside (and not just on walks), then they can develop behavioural issues; a bit like 'cabin fever' in humans. There are, of course, some exceptions, but the collie/sheepdog breeds in general really do need loads of stimulation in my experience, and unless you have the space, they're going to give you problems. A cocker spaniel, however, wouldn't need quite so much exercise and as they are members of the gundog breed, they are very biddable, easy to train and make good family companions. If you would prefer a larger dog, maybe you and your mum could consider settling for a larger gundog breed such as a Labrador or Golden Retriever; they are all very biddable dogs with nice gentle temperaments when treated correctly. I have a Flat-Coated Retriever; she extremely lazy, but without doubt the gentlest dog I've ever known.
With Shelties, I'm really sorry to have to say this...but unless you have a really big garden, they're going to need a lot of work.