I have just checked what information I have on three Foyle lines, the "Wardour" line constructed by Peter Williams, the "Foyles Of Beenleigh Abbey" from Burkes Landed Gentry (1982) and "Foyles of Chard and Chodrell". These include the bookshop people and cross reference also including the Foyles at Kimpton. These records go back to around 1500 in some cases. I could not find Juliana but she may well be one of the junior siblings along the way who consequently gets little recognition.
From what I gather, only the first son inherited the estate and his was the family recorded for posterity. The second son usually became a gentleman (soldier, rector, etc.) and had some initial assistance from his father. The lower down children got very little and largely made their own way but there were always exceptions I guess.
The Foyles of Chard and Somerford Keynes died out long ago.
Regards for now, Roger Foyle
Sometimes the younger children are not even mentioned in printed ancestry lists, and even these days, one name studies omit females. What is your source for the information about her position ?