I really adore both!
Anastasia is beautiful, and
Anina is great, I've never heard it before! I also love
Anya.
I really don't think the 'origin matching' matters, a nn should be whatever it turns out to be. I mean, look at mine for example:
Braelyn nn
Lulu. Thinking about it, it doesn't make any sense! But after a person gets to know me, tthey understand why. So weather it's sound or looks or personality or WHATEVER, it will work. (In my eyes)
Also, if you love
Anina, why not use it as a fn? It dosen't seem like it is strictly a nn. What about...
Anina Jane?
Anina Lilac?
Anina Katharine? After this post, I am SO adding
Anina to my list by the way! :)