William ap Yevan - William ap Yevan

I am curious as to the source that confirms William's wife as the illegitimate daughter of Jasper Tudor. If she *WAS* Jasper's daughter, I can trace my ancestry way back into the French nobility!

We don't know for sure. One thing is that two Welsh orphans got treated exceptionally well. Morgan ap William got property(became a well- known brewer,innkeeper, and even a lawyer) and married the sister of the Earl of Essex.John ap William was knighted and became a member of Parliament. They could never be Jasper's heirs; but, someone took very good care of them. They had to have some relation to nobility or royalty. One of Morgan's sons, Richard became one of Henry VIII's favorite knights and took the name of Cromwell. Oliver Cromwell descended from this man.

Data of Wikipedia isn't always correct, but the following should be considered:
Illegitimate issue[edit]
Jasper reportedly had at least one, but possibly two, illegitimate daughters by Mevanvy Verch Daffydd (1436-1485):[17]

Joan Tudor, (b. Snowdon, Wales c. 1453, d. 1469), wife of William ap Yevan (son of Yevan Williams and Margaret Kemoys), and mother of twins Sir John Williams and Morgan Williams (born Llanishen, Glamorganshire, Wales, 1469). It is believed that Joan died young during the childbirth of her twin sons.[18] Sir John Williams was the father of John Williams, 1st Baron Williams of Thame, who was the father of Isabel Williams who married Sir Richard Wenman and is thereby ancestor of the Earls of Leicester (7th Creation). Morgan Williams married at Putney Church in 1499, Norwell, Nottinghamshire, to Catherine or Katherine Cromwell, born Putney, London, c. 1483, an older sister of Tudor statesman Thomas Cromwell, 1st Earl of Essex. They were fourth-generation ancestors to Oliver Cromwell, meaning Oliver Cromwell was a descendant of the Welsh Royal Family, via Joan Tudor's grandfather, Owen Tudor.
Helen Tudor (b. Wales c. 1459, d. bef. 1485), wife of a skinner, William Gardiner, of London (born c. 1450), having by him: Thomas Gardiner, Prior of Tynemouth and four daughters, Philippa, Margaret, Beatrice and Anne. After her husband's death she married William Sybson. 19th century genealogists mistakenly conflated Thomas Gardiner with Stephen Gardiner, bishop of Winchester.[19].
Any thoughts?

I have found also he was married to Joan Tudor de Yevan
in my family tree.

There is a long discussion about Joan Tudor at Joan Tudor

Long story short: there is no contemporary evidence of the existence of Joan Tudor, though there is for a different daughter of Jasper Tudor.

The Welsh Genealogies -- which are the gold standard here -- not only do not mention her, but have Jasper connected to a person who is not Mefanwy verch Dafydd, who does not appear in them either.

The Wikipedia article, if you go to the sources, does not offer evidence of her existence.

It appears that the Cromwells invented her, as a way to connect themselves to the Tudors.

Not a daughter of Jasper Tudor.