Is OTHER wrong in the post ? POST : So this is my first post ever, and english is not my mative language, so sorry in advance. 

Background: Some time ago, my 16 year old sister (S) created a fandom account, after that, she started uploading videos to YouTube and she has done really well, her videos have views and now she has like 23k subs. She likes to go to her idol's events, although the problem with this is that she must travel 4-5 hours each time she goes to an event because we do not live in a city near the capital.

Since she does not work, everything is paid by my mom: tickets to events, bus tickets, meals, hotel, etc; all this with the justification that "it is not an expense, it is an investment".

On the other side, my mom (M) has been a single mom for several years, she takes care of all our expenses (incluiding me, my grandmother and my sister), which is really hard because the situation is not easy. Sometimes she struggles a lot for economical reasons and she limits herself to several things.

A few months ago, S got it into her head the idea that she wanted to make a trip to Argentina (we are from Mexico), she said she wanted to go there and that her idol was going to go on those dates. A couple of months ago S convinced M to take her and at the end of February they made their trip to Argentina for a week. I was happy for S because it was something she wanted for a while and I had never seen S so excited about something. The only detail with this trip is that it came out too expensive.

This is where the problem starts: A week after they arrived from Argentina, S told M that now she wanted to go to the VidCon in LA this July! I objected because its really not fair for M to have so many unnecessary expenses, since they have just arrived from Argentina and now S wants to travel again. Surely someone is thinking that I am envious or jealous, but it is not like that bc I was also invited to that trip. 

I did not stay quiet and talk to M, I told her things like: "Why are you worrying about unnecessary expenses?" "you two just went to Argentina 3 weeks ago" "at least demand her to do something, she must earn it" (S does not make any effort for her things, doesn't attend school as she should, and has even said that she is not going to study for her college admission exam the next year bc "i will not pass").

S was there and obviously she got mad at me, but I just feel that she does not care that M gets so much debt that she does not know how to handle the debts. On the other hand, M at the beginning  got upset with me and told me that nobody tells her how to spend her money (although she later got on my side)

AITA?
