Her great-grandson, Sir Henry Marney KG put a quartering on his garter stall plate in St George’s chapel, Windsor which is said to be that of Venables and is:

Sable, 2 bars Or

Possibly this might lead to identifying her father.

And, indeed, the Dictionary of British Arms, Medieval Ordinary, Volume One, p. 20 does have this entry:

“VENABLYS WB IV 181, 997 (name struck out; tinct of field corrected to ‘aseur’ in later hand.) “

WB IV is Writhe’s Book IV, c. 1435 and seems to be held in the College of Arms.