I saw something I shouldn't have seen and now I'm really hurt

Hey everyone, I was hoping you could help me out, my heart is breaking...

I come to visit my sister once a year because she lives in a country half was across the world, and it's very expensive to fly out more than once a year. She has been living in this country for about 4 years now, and I come to visit her and the rest of my family (cousins and grandparents) every year because I miss them terribly and cannot wait. She's the older sister by the way.

Anyways, this year I had a really hard year academically (grade 11) and it really took a toll on me (to the point where Physics made me have a full on screaming panic attack). I was really losing it, and it was really hard for me to make it through each day with a smile on my face during second semester, but my light at the end of the tunnel was the flight to visit my family and sister, and that's what kept me sane throughout my insane semesters.

I'm not going to write a book here, but long story short, my sister has two babies which are not easy to take care of, and I live with my sister and her husband so I can help. You have to understand, I'm giving up my whole summer (two months) to babysit, all out of love for my sister because I know how hard it is to raise children, especially when the husband doesn't really help and she stays at home alone with them all day.

Just so you know, I'm a teenager (17), so babysitting 24/7 is really difficult for me, especially since I can now see my cousins and have fun with them, and since I'm basically spending my summer days playing with my nieces, which sounds fun, but is actually a lot of hard work.

So today I was sitting in the living room with my sister and her husband, and she was speaking with him so I took her phone and looked at some messages exchanged between my aunt and my sister to see how my mom's surprise birthday party plans are carrying out. When I exited said conversation, I saw one that caught my eye so I clicked on it. Basically, my sister was asking her friend for ideas on what she can do with me this Saturday, also mentioning that I "sort of" help her and that she doesn't have the patience for my visit anymore (clearly stating she probably can't wait for me to leave already).

I was so angry, but mostly upset and I just don't know what to do anymore! I cried my heart out! I understand she's a full time mom and she wants rest, but when she finally gets a day off she can't even go and do something with me?!?! I wait all year to come and see her and go through hell at school to come and babysit all day and not even get to spend any alone time with my sister?!? I'm seriously considering working next summer without even coming to visit, that's how hurt I am.

Please help me out, and she's not a bad person, she's just not thinking about my feelings and it hurts me really bad.