The more I read your posts, the more I realize that Serena/Usagi deprived her mother from the experience (and joy) of being a grandmother.
Thanks for the words Clow! As to our girl...
I didn't think about it when I first saw SM back in the 90s. I grew up with DiC and they never mentioned "reincarnation" and even if they had, I wouldn't have even known what the word was. They said "reborn" or "Sent to the future on Earth" so I thought she was saved from the brink of death and then retconned into an existing Earth family. Cuckoo-style brood parasitism like Rika said. So of course Serena never told her Earth parents! Ikuko isn't her real mother, just some random woman who was hypnotized into thinking she was!
Of course that's not what happened. Serena/Usagi was indeed reincarnated and lived a whole life with Mama Tsukino as nothing else but her daughter. Ikuko loved her baby girl and carried her, she has just as much right to call herself mother as Queen Serenity did. I was always close to my own mother and yes, my favorite part of magical girl/child hero stories was the parent rooting for her kid. Thus, the more I look back, the more I ask, why didn't SM just tell her Earth mother?
Look at Winx Club! Sailor Moon's lookalike in role and function, Bloom, told her parents in the first episode. True,
the muggle foster parents are of no use in helping her be a good magic girl but if Bloom is morally good at all, it's due to them! Heck, Dorothy told Uncle Henry and Aunt Em back in 1910 in
Emerald City of Oz. After the damage the tornado caused in the first book, Uncle Henry was no longer able to pay the mortgage on the farm and it was their dimension hopping hero niece who invited them to come with her to Oz where she had since become a princess.
Don't believe me? Look down at this clip from a comic adaptation. We never see Serena/Usagi do this for the Tsukinos, do we? She instead chose to hide the biggest secret of her life from the people who loved her the most, the people who had the most right to know.
Girl, you might think you have reasons, but you are 14 years old, and they are your parents! How can you not tell them!?