I wouldn't be bothered by this for several reasons:
(1) The girls would be ten years apart;
(2)
Elena and
Helene sound pretty much different to most people; and
(3)
Elena would be a fn while
Helene would be a mn.
Your SIL can obviously do as she likes avoiding
Isabelle, but I don't really see a problem with this. And even further, I could even see myself using
Elspeth and
Isobel for sisters if they're both honoring the same person (or people). It's subtle too (like
Caroline and
Charlotte or
Nicole and
Veronica) and only name nerds would really catch on. It's not like you'd name the girls
Vickie and
Victoria (I've seen this) or
Lisa and
Beth. If the idea really bothered you, you could always use
Elspeth and
Isobel as mns. BTW,
Rhianna is really nice. I never understood the Rihanna spelling.
Re:
Eira, I like the way it looks and its meaning, but I'm always unsure of the pronunciation. I really dislike EYE-ruh (basically,
Ira).