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 perros that actually like to work. If they are not stimulated enough por 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 tu have the space, they're going to give tu 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 tu would prefer a larger dog, maybe tu and your mum could consider settling for a larger gundog breed such as a Labrador o Golden Retriever; they are all very biddable perros 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 tu have a really big garden, they're going to need a lot of work.