Thank God we can choose our friends because we are "stuck" with our family like it or not!!
Take it with a "grain of salt", have you ever thought that you may remind your sister in law of more interesting and freer , times when she was successful in the business or college world and not "just a mom and wife". Is it possible she resents your freedom (her perception) even if it is not true?
All families have their quirks. I think it is more common for in-laws to not get along than the opposite, for no particular reason. Really, go on and enjoy your vacation and if they are a part of it wonderful, if not have a good time anyway. It sounds like the shear distance between you may also have contributed to you both simply not having much of a connection with each other and she is less likely to entertain a change in that situation than you are. And remember "blood is thicker than water".
My step daughters now both adults live a few provinces over from my husband and myself and usually I pay for them to fly home for the holiday season. I have always been a terrific, caring, concerned and contributing step-mother in their lives. The eldest daughter turned 18 when we got together and was supposed to move to Europe but didn't and moved in with her sister who lived with her mom at the time. I have been with their dad 10 years, 9 of them married and I seldom get the time of day from them even when they lived near us. When they fly to our city for a few days or a few weeks (trip almost always paid for by my air miles +) we seldom see them for more than a quick get together over coffee and if we are really lucky lunch. They stay with their mom, have dinner with friends and other relatives, visit and spend time with everyone but us. They love their dad very much, count on him and call him frequently when they are back in Montreal, and they tolerate me no matter how much I tried to change that situation. In person we seldom connect. I gave up feeling hurt years ago and just take it for what it is. That has been really freeing for me, and I no longer feel rejected or hurt as a result.
Have a happy holiday my friend and enjoy life anyway including your visit with your parents .