You should count 24 months from when your youngest was born. If there was a leap year, then that comes out one month before her Hebrew birthday.
In order to uproot a veset kavua, one must perform a bedikah on the day of the veset for 3 months in a row. If the bedikot come out clean and menstruation does not begin on that day/onah, the veset kavua has been uprooted. Even if 3 months have passed and one did not begin menstruating on the veset date throughout the 3 months, if a bedikah was not performed on each veset day, the veset kavua has not been uprooted.
Therefore, if you had a veset kavua for every 4th Shabbat at 11:30, for the next 3 months you should perform a bedikah at 11:30 on the 4th Shabbat from your previous period. If your bedikah on the veset is acceptable for the next 3 months, your veset has been uprooted.
Nevertheless, unless a new veset kavua is established, the old veset kavua remains dormant. Meaning, if in the future you get your period on the 4th Shabbat from your previous period, you have reverted to your old veset kavua, and must perform bedikot on that veset for 3 months until it has been uprooted again.
The halachot of veset l'yom hashavua (veset on a specific day of the week) are complicated, since the veset itself is similar to a veset haflagah (interval). Should you revert to this particular veset, please be in touch with us for further guidance.
Please feel free to get back to us with any further questions.
B'Hatzlacha!