No, Cecilia was not a daughter of Nest and Osbern. (You're looking at an unsourced web tree; anybody can say anything, and it's best not to use them for the historical tree.)
No medieval sources give the two a child named Cecilia --
furthermore, I see that the site that you link to says that Cecilia married Siward de Arden.
Siward de Arden DID marry someone named Cecilia, but she was not Welsh -- she was from an Old English family, the grand daughter of Aldgyth, according to Wikipedia, though they are not citing sources --
Shakespeareana Genealogica from 1869 says that Siward's wife Cecilia was a Drummond -- https://www.google.com/books/edition/Shakespeareana_Genealogica/rFw...
William Dugdale's Antiquities of Warwickshire, from the late 17th century, gives her name as Cecilia but does not give any family name -- https://quod.lib.umich.edu/e/eebo/A36791.0001.001/1:18.168?rgn=div2...
On Geni she's here -- Cecily of Mercia -- with no family name, but that she was from Mercia is in the display name, and that is, I think, the best solution at this time.
there are guesses as to who she was, but we don't know.
For sure she was not the daugher of Nest ferch Gruffudd.
Here's the
