Diana supports her

Seeing Grace is about to slip, Diana quickly crouches behind her, reaching around and placing her hands under her thighs to keep her from slipping off the chair.

"Thanks!" Grace breathlessly says, smiling over her shoulder at Diana before gasping again.

You decide it's time to escalate, and you slip your thumb under her panties, finding her wetness underneath. As you part her lips with your thumb she lets out a cry, and grabs the sides of her chair.

"You're doing great," Diana murmurs in her ear.

You rub up her lips toward her with your one hand, then the other hand joins the fray and you slip your second thumb deep inside her. Her eyes grow wide and she lets out a low moan. She leans her head back against Diana, who keeps whispering into her ear, lips brushing against ear and her cheek, nose almost nuzzling her. Diana seems to be- very invested in what's happening.

In a swift movement you pull Grace's panties down to her ankles. She nods furiously as she watches you to see what happens next. Without hesitating, you slip two fingers inside her, as far as they'll go. She cries out as you start finger-fucking her, slowly first, and then faster.

Diana is now kissing Grace's cheek, down her neck, first reassuringly but then with increasing passion. Grace is fully leaning into the kisses, legs splaying for you, gasping and moaning with greater and greater volume, sounding almost plaintive, as if she wants something that she's never fully experienced.

Suddenly her head shoots up from Diana's chest, eyes wide open, gasps frozen in her chest for a moment, the only sound the wetness of your fingers sliding in and out of her. Then she cries out, loud and full, gasps three times in rapid succession and then screams again. You can feel her pussy contracting tight around your fingers, pulsing for long moments, and then finally releasing into warm wetness.

Grace takes a minute to recover her breath, gasping and then breathing and then sighing, her body slowly relaxing as you slow your ministrations. Diana leans back but doesn't let go.

Looking down at you, face flush and intent, Grace says, "Now I understand. Thank you."