I know it really hurts.... and after all you have done as a loving sister/Aunty.
But well, sometimes, people can make other plans... with others.
And its okay.
I am sure your sister did not mean it as a personal affront to you nor to hurt you.... unless she is typically a 'mean' person toward you.
How is your sister overall, toward you? Nice? Appreciative? Kind? Thoughtful? Respectful? If so, then I would just try not to feel offended about this trip.
Sometimes, roads just diverge for the moment.
I am sure they both really love you... and know what a big influence and help you have been to them.
Maybe when they get back, you can all celebrate in another way, and have time together......
Also, your Sister just reunited with her old high school friend... .so this will be a "honeymoon" sort of thing for now... it is still new and fun for the time being reuniting with a former friend... if you feel she ignores you then... then speak to her about it.
But all people, have 'social lives' aside from their family too. Which is fine and to be expected. No matter how close you are with each other. People have other friends and socials with other people. And sometimes they just want autonomy that way. Its okay. People deserve to have a life with other friends. Its normal.
They are texting you and 'sharing' their trip with you... including you that way. I don't think they mean to hurt you.... by doing that.
But I understand you feel left out and miffed.
Just talk with your Sister, kindly... and as a sister. I am sure she may not even know how you feel. But I do not believe, they did anything 'wrong.' There will be parts of their lives, that are spent with others....
all the best,