need advice.

Okay so ive been dating this guy for a little now, we met because we have a mutual friend ( im gonna call him B ) well B and my now boyfriend has been friends for awhile, and B had told me that my boyfriend slept w B's sister, ( awhile back not when we were together ) and i was like uh okay. and now im kinda mad because my boyfriend was still going over to her house with his friends ( like before we started dating but we were talking and made it clear to eachother that we want to be together ONLY US ) and he still has her on snapchat and he was just over at her house the other day ( with B ) , also she was buying him stuff for awhile ( while him & i were talking idk if she still does ) i just dont know what to do. should i be upset? am i overreacting?

Edit:

Im 16

boyfriend is 17

B is 16

B's sister is 22 or 23.