stomach flu!

So my sister and brother in law and their kids  came down with a terrible stomach virus over Christmas, they live out of state and came Into my in laws house on Tuesday after they were "all better". We went to their house on Wednesday evening for dinner and present opening. My 3yo was excited to see her cousins so she spent the night. My mother in law texted me yesterday (Friday) and told me she woke up at 1:00am violently ill. I'm freaking out thinking my 3yo will get it. It's been well over 48 hours since she was at their house.   We wash hands well but I'm sure it was a bit more lax at their house when she was there bc it was crazy with kids. Please help!