Hmm, she may have been living close to Leeuwarden, but could have been born somewhere else entirely. So lots of uncertainty here. Anyway, may I suggest a married couple as candidate parents for Sibbel, also based upon earlier findings/discussion here about the Andringa's? So this is an hypothesis. No proof whatsoever yet.
Looking at your statement above, and also the data collected in this publication, the eldest daughter of Sibbel and Rienck was 'Antje', suggesting that that could be the first name of the mother of Sibbel.
Which immediately boils down to the marriage of Lyeuwe Bauckes (living in Marrum) with Anna Leuwis dated 07-06-1595, Gerecht Leeuwarden. 3 reasons in favor of this marriage:
1) (Of course) the expected first names of the parents
2) (Of course) the date of the marriage is 'in range'
3) Civil marriage which would be consistent with 'Doopsgezind' (see previous string)
Marrum is not 'onder de klokslag van Leeuwarden.' But they married in Leeuwarden; no way of telling (yet) where they may have settled. There is a couple of other marriages that may fit the 'picture' here, but these are a bit 'late' (as in 1611/1612 leaving little room for Sibbel to be born) and are DRC-church marriages.