I am not trying to be rude, I really am interested in the answer. Are showers for second babies normal in your circle? The reason I ask is that you really do not seem to need anything, you are having a baby of the same sex in the same season and are challenged for space.
Under these circumstances I would think showering for the second baby is a little overkill unless your family celebrates every baby this way. People will bring a gift when they visit if they choose but an invitation is not easy to get out of. I know folks say "Just decline." but that is not always as easy as it sounds and asking people to bring gifts that you do not need and really do not want could be misconstrued.
I think that if my Mom really wanted to celebrate the birth of a new baby I would ask her to make it a party and make sure that she let people know that you do not need gifts. This would take the pressure off of family and friends whose finances are as stretched as yours are.