Sisters are the easiest and hardest person to buy for. Easy because you know her; hard because you might be buying what you would want.
Choose for her wardrobe, not yours. If she wears gold and you wear silver, buy gold. If she never wears earrings, do not start now.
Should we get matching pieces?
Only if she is the kind of person who would wear it. Matching jewelry reads as sentimental to some and as a costume to others. If in doubt, buy two pieces in the same family — same metal, same weight — that are not identical.
What works for a sister I don't see often?
Something small enough to wear daily so she thinks of you often, rather than something for occasions she might not have. A thin chain or a pair of small hoops does more than a pendant she saves for best.