That's really interesting! I hadn't considered Labradorite's element before, but I have a piece with a lovely blue flash that looks like a pocket of ocean inside.
Looking at my family's elements, I'm Air, but my sister is Earth. My mom is Fire, and my brother is Water. My brother works well with Labradorite (and trust me, he is Water), but my sister does not. Hmmm... very interesting...
My position on working with the Elements is more complex than simply saying that someone "is water".
People's Astrological Natal Charts are very nuanced, with the Sun, Moon & Ascendant being the "power players" (but other planets potentially being equally important, depending on their placement & aspects).
So, for example: if my Sun is in a Fire sign, my Moon in Water, and my Ascendant in Air....what Element "am" I?
Even if you were to determine by looking at my Natal Chart that I have very strong Fire energy, this doesn't necessarily mean that I need more Fire in my life -- perhaps I should temper all that Fire with Earth, extinguish some with Water, or feed it with Air?
Therefore, I prefer to direct my exploration of Crystals from the position of "what is missing from my life / what Element or Energy do I need more of?" vs. "is there an excess of any Energy / do I need to balance anything out?"
From this perspective, if your sister does have a very strong Earth influence in her chart (besides her Sun sign being in Earth), perhaps the Labradorite is too Earthy for her, its effect too grounding -- where she might need an infusion of another element, to balance out all that Earth.
Having studied Tarot, Numerology, and Astrology I eventually learned that, while we often tend to say that someone "is Fire" or "is a number 9", it is generally more useful to think in terms of someone "having lots of Fire energy" or "working with the number 9".
Back to the Labradorite: it may very well be that both you and your sister are having very emotional reactions to it, as people react differently to various stones (while personally, I find Labradorite very peaceful and soothing).
But if you don't mind me asking, have you Cleansed, Cleared or Charged that Labradorite recently?
It is also possible that there is some residual energy attached to the stone, which you & your sister are picking up on...