My old best friend from my teens and twenties did that to me with her engagement ring though, syk and I was a bit 
She really did have form for blatantly copying me, for example she bought virtually identical sofas and curtains which were not at all a common style or colour at the time. I found it irritating but I knew I should be flattered deep down that she just admired my taste and mimicked it. I am very into interior design and quite well known among my friends for rooting out unusual things or adopting colour schemes well ahead of the masses so I am flattered when people are influenced but I'd prefer them not to be quite so brazen about it! Either that, or to totally brazen it out and admit they loved it so much they couldn't resist copying, which I would accept with good grace.
I got engaged about 18 months before her, and her ring was a carbon copy of mine style wise, but the diamonds were twice the size! It was not an especially popular or common style at the time either, so it was pretty obvious she'd been influenced by mine. My diamonds were much better quality than hers but I couldn't help feeling a bit wound up by it. I just plastered on a big smile and said 'ooh lovely! It's just like mine but bigger, you lucky thing!' and then did a silent sigh and a secret eye roll at DH.
Then when I was PG I told her my name choice for a girl. She never made any particular comment about loving it at the time, I'd never heard her mention the name before and this was the sort of stuff we'd discussed a lot over the years, as you do with your bestie, but while I was still PG she started TTC and lo and behold, guess what name she wanted for a girl, and said it had apparently been her favourite for 'years and years'? 
Because of the risk of history repeating itself I called my DD something else entirely in the end, very last minute hush hush choice, but she had a DD shortly afterwards and did indeed use the first name I'd picked.
With your sister though syk some people can get very very shirty if they feel they've been upstaged in any way. Particularly if it's a sibling who always feels insecure or second rate around you for whatever bizarre trumped up reason in their head, it can be really tough having to walk on eggshells your whole life for fear of doing something they perceive as a deliberate act of oneupmanship.