With me, it seems like the harder I'm trying, the blanker I get, until literally the cards turn into some Dead Language that just stares at me intimidatingly, taunting me. When the cards are speaking to me in some foreign, archaic language that I don't even BEGIN to understand, how on EARTH am I supposed to relay that to my sitter?
My solution is to step back. And I mean really, really step back, separate myself from the entire situation mentally. Do you have a favorite place, one that makes you feel calm and at complete peace? Imagine yourself there, immerse yourself fully in it. Feel the sun on your face, the water lapping at your feet, hear the crickets chirping, whatever it is. Once you're centered again and feel more balanced, try looking at the cards again.
What's changed now? What seems different? Does anything stand out as significant? Remember, you don't always have to read the cards in the order in which they fall; if something really jumps out at you as important, start there and then web your way out into other cards.
As nisaba said, it can be incredibly hard to read for people you know, especially those that you know well and have a special attachment to. You really have to be able to compartmentalize and put aside whatever prior knowledge you have of the sitter and their situation. This in and of itself is immensely difficult, and there's no shame in not being able to separate the two. Nisaba said it best; sometimes, your intuition and your knowledge of the situation just overload and short-circuit and you just go completely blank. If necessary, you can always refer your sister to another reader, or even have her join here and participate in exchanges! We do have the Newbie Exchanges, so that might be perfect.
Another thing you can try is simply starting with one card. This is another personal thing, but let's say I'm doing a 5 card reading. If I turn all 5 cards over at once, I easily get overwhelmed and can't see connections between cards. If I started with one card, then turn over the next, then the next, etc., I find I'm able to "tell a story" easier that way. Draw the Heirophant, describe what that means to your sitter, then turn over then next card, say, the 5 of Cups, and draw correlations between the two. Then pull turn over the next card, let's say...3 of Cups, and continue linking them together and you go along. Just something you can try and see how it work for you.