Mikuru got it from her grandma (Director Mafuru) Ig, and then if Mikuru is really Anna's mom what if the watch that Anna saw on the table in her birthday was from Mikuru. Then, Decchi Agein mention why there is two Jewel cure watch, my theory is the one Mikuru had is from the past and Anna's one is from the future.
She said that it has a memory value to her. If she's Shushutan, what if Cure Éclair gave it to her and there's a reason behind it. Then, If She's Cure Éclair, then my theory is that it is her transformation item or something valuable since her memory is lost it's just all she can remembers is her earrings.
I'll just repost it here
I know that if Shiny luminous is in Splash star, it will create a big impact on the story of Max heart. But, for me her design is nearly identical to the Splash star group making it seem like she's part of them. plus, if she joins them the Fu**tari wa precure will stay as duo and max heart precure will not exist.