How bad is to be dating with an under age person???

I found out that my sister (she's 16) it's dating a 20 years old man!!! I'm so disappointed by her, she posted on Facebook and when I asked her about, she acted like she didn't know what I was talking about. I haven't talk to my mom yet, but I know that she doesn't care, one of my sisters was in the same situation a few years ago and now she has 5 kids!! All of this is happening in Mexico, I was so excited to visit my family for the first time in a year and half and I now I don't want to go. I just can't believe that my mom it's fine with this situation, she never really care about us, but why?? I'm the big sister and I always care about my sisters more than my mom cares. My sister told me that age doesn't matter, that she won't end like my sisters... I'm pretty upset.