Every day

You decide to strike a deal with Brittany.

"Meet me in the bathroom at 7:20 every morning and I'll give you your daily dose."

"What if we get caught?"

"I can go back to the tissue."

"I'll be there." She pouts.

The next day, 7:20 on the dot you're waiting in the bathroom, the doorknob turns and your sisters ducks inside. She's wearing her dressing gown, revealing beautiful cleavage within.
You're already aroused with anticipation and she drops straight onto her knees when her eyes lock on to your manhood.

Eagerly she goes to work sucking your cock, cupping your balls and working her tongue along your cock head. She lets her gown drop around her shoulders and more of her breasts are exposed.

They are so perfectly round, milky and pert, you reach down and squeeze one affectionately.