What happens to petitions when I become US Citizen

[ChelleS 0]

My dad just started a petition(F2A) for my sister (she'll be 20 next month, unmarried, and living outside the US). Because she will be turning 21 in a year, my dad is deciding to apply for US citizenship (normal processing time is around 7 months in FL) to "freeze" my sister's age (CSPA law). He's always been wanting to apply for citizenship, so I think this is such an appropriate timing. Anyway,  we received a priority date for my sister's petition of January  2020. Right now, Visa Bulletin is processing applicants with priority dates of Novermber 1, 2020 from the Philippines. It's looking like, and we pray, the petition will be current, approved, and sent to the NVC sooner than expected. I mean, we are being optimistic here, however, we are confuse as to what would happened if for example, my sister was scheduled  for the consular interview abroad and then my dad naturalize on the same week or day? Will this delay everything? Do they need to change the status? DO they go back to step 1? Who and do we notify anybody? the interviewer, the nvc, who? 

Also, What does below 21 years old really means? Does this mean she needs to be below 21 during the approval of the petition? Is it below 21 during the consular interview? Or is it below 21 upon her arrival in the US? I know she will receive a visa after the consular interview, but will not actually receive her greencard until she gets to the US? Is this correct? I believe as soon as she get to her Port of entry, the immigration officer at the airport will stamp a greencard on her passport, correct? So going back to my question, does below 21 means, she must fly to America and receives her stamped greencard aon her passport  before she turns 21?

And I know this is a stupid question, but she is actually a leap year baby. So when does she actually turn 21? Feb 28 or March 1. LOL

Thank you again.