I say still go with [name_f]Isla[/name_f]. [name_f]My[/name_f] cousin, born 8 weeks after me was given EXACTLY the same first name as me. [name_f]My[/name_f] mum was really upset at the time apparently as it was her first baby and so proud of the name they’d chosen. When my cousin was born 8 weeks later and given the same name, my aunt told the family that it was the name she’d planned on giving a daughter ever since she was child. The family accepted it and everyone moved on. As for me and my cousin, as kids we actually thought it was pretty cool that we shared the same name.
Since the name they’ve chosen isn’t identical to [name_f]Isla[/name_f] I say go with [name_f]Isla[/name_f] anyway. The cousins will probably like having similar names, the parents will get over it.
If you feel like you can’t go with [name_f]Isla[/name_f], have you considered [name_f]Eira[/name_f]? It’s a welsh name that means snow and is pronounced eye-ra. I think it’s really pretty and definitely shouldn’t be one that anyone else in your circle might use!