If possible, go for one + fusion. It has a full hd screen, 6 gigs ram so you are future proof and also it would help her in consuming content(studies and videos). I do not know the use case for your sister but generally speaking from a long term perspective for a mobile (2+) years.
It's a good device, both fusion and fusion+. The later is more future proof, or atleast, those were differences I observed (mentioned in above post). I would have suggested Nokia too, but given the number of problems associated with most of their device lineup's, it would be better to stay away from them.