In my school growing up, we had twins a year younger than myself named
Ylva and
Edda. I honestly adore that twinset, I like the simplicity to both of the names, and I think they fit well together without really being similar at all.
To be honest, I prefer
Ylva over
Alva, I think it is about having two different vocals in the name, it just sounds more delicate to me. But I do think
Alva is also very pretty, and I like how "alv" means "elf" in Norwegian, it just makes it a bit more magical to me. And the word in the name doesn't make it weird either, because having "alv" in names has always been common, as we also have old names like
Alvar (m), Alvilde (f), and even just
Alv (m), where in the last one the variant
Alf is probably much more common now.
Ylvie sounds very sweet, but is better for a nickname in my opinion.