My sister (18) is here in United States on student visa. My husband and I pay for most of her stuff, in case that matters. Her boyfriend is Mexican citizen and she was telling me how she wants to go with him to Mexico during spring break. I told her that currently Mexico is not safe, and even my Mexican co-workers are avoiding doing there. I told her that I would love to visit Mexico too, but right now it's not worth the risk. We left it at that, no big deal. 

Earlier today, I get a text from her saying that she is going to Tijuana for tacos with her boyfriend's family. I called her and asked whether she can enter the US back, and she said that his family 
said it will be fine. I informed her that boyfriend and his family are Mexican citizens; however, she is a citizen of different country and she will need visa to get it! But most importantly, will she be able to get back into the country? She kept saying it'll be fine. I told her that this decision is irresponsible and she needs to come back home, to which she replied "it's going to be fine." 

I broke down crying because this is not the first time she makes an irresponsible decision and goes against me. My husband called her and told her that if she doesn't come home right now, she is flying back home.

She called me and I explained to her that right now she is my responsibility and I am her mother figure. Our mom (who is 10,000 miles away) is sleeping right now and doesn't even know this is happening! If something happens to her, it will be on me and I don't want to risk anything happening to her. She told me I am overreacting and start acting like sister, i.e. support her. I totally lost it and said that she is 18 and this is her decision. However, if she chooses to go to Mexico, when she gets back she needs to pack her stuff and find a new place to live because clearly she doesn't give a shit about my boundaries. She replied that after my husband yelling at her, she doesn't know if things can go back to normal. 

Our family shares our locations on Google Maps, and she is by Mexican border, so she chose to go. According to my Google search, she does need visa to get to the Mexico, AND she needs to notify her school prior to travel. I am so pissed at her for completely ignoring my concerns and presenting me with the fact that she is going to Mexico on the way to Mexico?
TLDR: Younger sister went to Mexico with her boyfriend's family and notified me on her way there. 