it’s totally fine imo! and this coming from someone who tends to avoid similar names to family members. the age difference alone would make this okay for me if I really loved the name, but even if it was just a year between them, I think I’d still use the name. I don’t think there’d be many instances where calling [name_f]Lu[/name_f] could be mixed up with calling a 15+ year old [name_f]Lucy[/name_f], if that makes sense.
[name_f][/name_f]A similar example from my own family: my aunt and uncle purposely chose uncommon names for my cousins. less than a year after my youngest cousin was born, my aunt’s sister had a baby girl and named her something which rhymes with my cousin’s name (think [name_f]Maya[/name_f] and Kaya). at first, my aunt was upset because the names were so similar (and the girls were so close in age) but the girls are 12/11 now, and everyone has survived lol. I don’t think anyone really cared once “Maya” was born anyway 