I'm getting worried here…

Okay, so three days ago my sister was very sick and vomited often, the next day she had a stomach ache but did not get sick. The day after that my brother who was also with us got sick and it was bad, but not as bad, he was in bed all day and I think only got sick once, they thought it was food poisoning since the bad part only lasted a day. But today my mom (who did not eat at the place where they believed they got food poisoning from) went to bed at eight and just recently vomited very violently and appears to be in a lot of pain in the stomach area. I am getting worried that this will continue to me and we don't know what this is. Even worse, the day my sister had it we had 75 people in that house. Many of around 50-70 years old (based on an estimate) and my sister and brother (he is my brother-in-law) have a daughter who isn't even a year old. My main concern is that we have something very contagious and may have infected many people. It would be very helpful if someone can help identify what this is. I am thirteen and I have no medical knowledge of my own. I would like it if you can identify this to link to a trustworthy source to prove your statement.