Hun, they didn't want you (more likely your daughter) at the shower. It's "last minute" in hopes you wouldn't be able to find a sitter, or that you would leave as soon as your daughter started her tantrum. She may have confided her struggles with your daughter to her MIL, so her MIL took the shower into her own hands because she doesn't want her to miss out due to her niece's/your daughter's bad behavior.
YWBTAH if you went with your daughter. Find a sitter for the day and then find a child psychologist to get to the bottom of this jealousy before it gets worse. We all know at the bottom is spoilage (spoiling a child ALWAYS backfires), but you need to learn how to set boundaries and stop spoiling. She's 8 and should know the world doesn't revolve around her at this point.