Sisyrinchium striatum (Satin Flower). There is also a variegated variety 'Aunt May'.

Thankyou

I agree. It self seeds easily so your sister may have to pull a number of the seedlings unless she wants it to take over.

Wow a blast from the past, hello Milky, long time no see, I have this one, yes it certainly does self seed rapidly but easily removed, lovely little dog you have there....
Jimmy thanks for the info, I have never seen the variegated one so must check it out...