Me my mom and my other 1 sister found out my older sis has a BF. IDK WHAT TO DO! ?

So yeah idk how to take it. I was very surprised by what happened. Shes 17 and im 16. Do you think shes old enough to be seeing someone. Should she get punished for not telling anyone for a year? I never knew and shes always been a liar to me. Idk what to think anymore is she hiding even more things from us? Please someone a swer this question i need to know. I dont feel comfortable when my sister is around guys.