I love Maren and Marena - both are lovely names. Have you talked with your sister about this? All of my children's name are after someone - either their first or middle or both but it is your sister's name and maybe she already has plans for it. For instance, my name is Keri and I have a DD named Kerianne I would have been pretty pissed if my sister used my name without my permission. As it turns out she's not having kids so it's not an issue but my feeling is that she gets dibs on her own name. If she is ok with it then I say go for it! My oldest is named after my brother and I am so glad that I did that! If you think the girls names will be too similar maybe you could use it as a middle name.
Keri
Keri