I'd like to announce I have seen through the matrix. Uncle was dancing with Hotaruko's mom, the woman of the trio, and the person who would become Hotaruko's father, the other male of the trio. Uncle had an unrequited love towards Hotaruko's mom. Maybe he gets flustered when he sees Hotaruko because she reminds him of her mom? Maybe he was around during Hotaruko's childhood because of this connection and she developed a crush on him?