Great Grandparents?
So if Henry and Celia Mack, who Andi thought were her parents, turned out to be her grandparents, did Andi ever have any alleged "grandparents" before she learned the truth on her 13th birthday?
Lauren Tom was born in 1961, so her character Celia's parents could have been born about 1921 to 1941, and thus been about 63 to 83 when Andi Mack was born in 2004.
Thus it seems possible that Andi could have met one or more of her alleged "grandparents" on the Mack side of the family as a younger child. It would certainly be possible for one or more of those alleged "grandparents" to even be alive in 2019 aged about 78 to 98.