Unexpected First Threesome

I was at my girlfriend’s house Saturday night keeping her young sister so her parents could have an evening out.
After her sister had gone to bed we were in my girlfriends bedroom having sex when her sister walked in on us.
When she realized what we were doing she told my girlfriend she was going to tell her parents.
I was surprised when my girlfriend told her she’d let her join us if she wouldn’t tell. She hesitated but then jumped in bed with us then we continued what we were doing.
She just had a tee shirt on over her panties so my girlfriend started playing with her tiny boobs
My girlfriend whispered for me to touch her between her legs to see what she did. We chuckled when she started moaning.
Once my girlfriend and I had finished we stopped then her sister said she wanted to do what we had been doing.
My girlfriend is on the pill so I didn’t have a condom but she told me to just put it in her for a minute then pull out.
My girlfriend told her to take her panties off then started to finger her but told me she was already horny enough.
I couldn’t believe how wet she had gotten from the little we had done to her but she was still very tight.
When she actually move a couple of times I pumped a few times and before I knew I shot my load before I could pull out. Thankfully she hasn’t had a period yet so it shouldn’t be a problem.