It sounds like presents and not presence to me.
One, it is incredibly odd that she's having a baby shower after two children. I always thought you got one (or them in some cases) for the first and only the first unless you had a child much, much later on. Two, you said it's her mother who is throwing the shower? While she may not know, but immediate family members aren't supposed to throw those types of things becuase it seems like a solicitation for gifts. It's like you're mother throwing you a bridal shower.
Maybe the want to celebrate the pregnancy? Are they an infertile couple that has been really lucky on InVitro (although one a year ago kind of forces that out of the question)? Maybe she's having this one and the last one too close together and they are strapped for cash and thought this would be an easy way out? Doesn't make it right, but I'm sure it's not the first time it's happened.
Something smells like week-old fish...
ETA: Just kind of mulling this over. I think it's nice for families to have celebrations for each of the children, but it seems like it should be close friends and family and likely after their born. Maybe your sister is considered a close friend and has fallen into a select group that have been invited? Still doesn't make right the registry, unless you consider that it might be for family and whatnot. I know when you register for a wedding, they keep it in the system for awhile. For my Birthday, a number of my family members sent along gifts from our wedding registry (December 2005 wedding) that were still left unfilled. Registries are tough as if you don't register people assume you're doing the, "We want money game." If you register for what you don't have, and in many instances couples live together before marriage and have the necessities, you're playing the, "Buy us expensive gifts we wouldn't otherwise afford." If you register for everything cheap, it's an insult to the guests. If you don't register for enough gifts, people get upset, too. I think they're miserable things!