Her official title says "The Secret Sealing Club's First President", so she's, perhaps, Renko's older relative? That is, if there were other club presidents besides Renko.
Same kanji, actually. Renko is 宇佐見 蓮子, while Sumireko is 宇佐見 菫子. There is only one odd one.
...Oh, well, shoot. Checked again, and you're right, nothing's different last name wise. ...Wonder what the story behind this is going to be. I'm pretty sure this is the first time the Sealing Club's even been referenced to in a Touhou game, after all.