Well, congratulations to your cousin. Out of that list:
Amy - very pretty name, doesn't 'date'
Bryony - always think of Briar Rose in Sleeping Beauty
Cassandra - don't like this because of the original name holder in the Iliad - she was cursed to see misfortune, but never be believed when she warned people of it. Also came to a bit of a sticky end IIRC.
Cassidy - bit bland
Francesca - like the sound, and a 'stronger' name than Amy or Bryony
Ingrid - don't like it, but I knew one when younger that I found a bit of a wimp, so am biased.
Kayleigh - again, like Cassidy, bit bland
Matilda - always think of William the Conqueror's wife - she was not a nice person by all accounts...
Robyn - nice, reminds me of an Enid Blyton type though
Saffron - do like this one
Tegan - same type as Cassidy & Tegan
Zara - the shop and Z. Philips spring to mind.
For other suggestions, how about:
Cara ('beloved' in Italian or 'dear one' / friend in Irish)
Bridget ('the exalted one' - she was an Irish goddess of fire, poetry and wisdom)
Morgan / Morgana / Muireann ('born of the sea' - a Welsh mythological link this time though)