For my understanding, you can still apply for bto but you must sell your flat when the new bto key is ready for collection, till then I have already met me mop on the resale flat.
Correct me if I am wrong.
Theoretically, you are wrong. You must meet the MOP of the resale flat, before you can apply for the BTO.
It is clearly stated in HDB's website
here, which says:
HDB also lets you check your eligibility from
here, and a simple fill in the blanks would give the following results given your situation:
One example : My Sister have a flat which was joint name with my parents, she went to apply for bto. Pay her dwn payment few years ago and just sold their flat this year while she is going to get her new bto key this month.
Some info you missed out, is whether the flat your sis co-own with your parents, had the MOP met already, before she finally successfully get the BTO. Do check with her again.
You can apply as many times as you want, but that would only mean you are wasting money and time when MOP is not met.