My older sister found condoms in my girlfriend's purse?

Alright, awkward story. So, my 20 year old sister brought me and my 15 year old girlfriend out to dinner so they could meet each other. When my girlfriend went to the bathroom, my sister started going through her purse to look for a pen to sign the check because the waitress forgot to leave one. Before finding a pen, she found some Magnum XL condoms. She confronted me about it later, not thinking they were mine. She told me I should watch my back. I'm also 15 and live with my dad who doesn't know about us having sex. If I tell my sister, who can't keep secrets, the truth, she'll tell my whole family, including my dad and even my grandma. On the other hand, if I play along with what she thinks now, she'll tell my family that my girlfriend's a slut, which is 100% not true. What do I do?