Girlfriends Gullible sister

The Girlfriend has a Gullible sister, she always wants what my gf has, that includes me, thing is she is so Gullible, she offered her tiys to play with and jacked me off, over a few months we have done a lot more, all i have to do is say your sis & I do.... and she is up for it, thing like the gf ass is exit only, but i tell gf sis that i fuck her ass, she went on all 4's parted her ass cheeks, spit for lube and in i go balls deep, I know i am taking advantage but its so exciting