Sister likes to watch me and my gf

My sister likes to watch while I fuck my girlfriend. My gf and I are both 19 and my sis is 18.
The first time was when she walked in on us doing it by accident. She didn't know my gf was round at our house. She walked in the room and just stood there saying oops but continued to watch as I was fucking. It was weird but then the next time my gf came over my sis asked if we were gonna fuck and I said yes so she said she wanted to watch or else she'd tell our mom so we agreed.
We got down to it and my sis just sat on the chair watching as my gf sucked my cock while I was lapping at her bald cunt. Then once we started fucking I saw my sis had her hand between her legs rubbing her own cunt as fast as she could. Then she let out a moan as she climaxed. Me and the gf were getting turned on more by seeing my sis that we came together in seconds.
Now my sis says she wants to join us for a 3some. What do you think?