Yes

See how horny Allison's getting has you more than a little riled up too, and you find yourself going along with Allison's request rather than ruin the mood. You attach the photo and send it off too Fiona with no subject or text, keeping the screen tilted away from Allison to keep your sister's identity a secret.

"I-I sent it." You put your phone to the side as Allison fingers herself with more energy than before. "Fffuck, just thinking of him looking at my cunt gets me so hot!" She grips your hair tightly with her free hand, pulling you forwards just enough to let you know that she's ready for your mouth.

You don't need any extra encouragement, and eagerly extend your tongue to lap up her overflowing juices. Her body quivers in pleasure as you lick and kiss her slit, then slip your tongue inside. You feel a sense of satisfaction as she gasps, her juices seeming to flow without ending. The way her body tenses and quakes as you please her with your mouth is rewarding...it's your first time pleasing a woman to this degree.

After some time, you feel her other hand grab your head as well, and she moves your head while grinding her hips against you, as if fucking your tongue. You don't have to be a mind-reader to guess she's remembering getting fucked by your "friend's" cock, and push your tongue in as deep as it'll go to accommodate her.

She gradually grows more and more intense in her movements, and it's no surprise when she shudders one last time, cries out "fuucckkk!", and orgasms all over your face. You feel a sense of accomplishment as Allison sags back on the desk, looking very content. It's your first time making her cum like that, even if you had a bit of indirect help.

As you wipe some of the mess off your face, you realize you and Allison should probably get the hell out of there. You tried to keep things quiet, but at the end, Allison more or less yelled "fuck" at the top of her lungs; it wouldn't be surprising if a janitor or something swung by the room to see what was going on.

...After you point out as much, both of you make a run for it - while laughing -, splitting up for some washroom-assisted damage control before heading home.