Slowly rub your middle and index fingers in and out of his hole, letting the lubricant seep throughout his anus. Continue until you can get into a moderate rhythm of insertion.
Then take your own cock, which should be fully aroused from this sexualized context, and insert the tip of it into the now-slippery asshole. Slowly insert it, inch by inch, until the base of your cock hits his anus.
Pull out halfway, and thrust into him again moderately, and repeat under a steady rhythm. Make sure you grunt and moan whilst doing this; your brother should involuntarily be doing the same.
Kiss him. Kiss him hard as you thrust. Tell him how much you love him, how much he means to you. Suck his face, his neck, his chest, his perky nipples, caress his arms, back, torso, oh god, anywhere and everywhere. Pull back his hair, speed up your thrusts. Let the balls smacking into his ass fill up the silence of the room, let your voices drift in sync to your passionate love-making.
Hold him tightly, moan loudly, tell him how sexy he is while you bang on his prostate. Use either hand to jack him off. Match the rhythm of your fucking and masturbation. Eventually, you'll feel a hotness in your groin. It'll soon consume you, overtaking your consciousness, your soul. Let yourself go.
Cum. Cum hard. Let the semen shoot straight from your dick into his anus. Let the pleasure of the orgasm speak to you and hit you in waves. Your fucking and masturbating should also consume your younger sibling in pleasure, and he too should begin to jizz. Let it fly over his chest and land on his neck, face, abs.
Breathe, and collapse on top of him, out of breath. Ensure he is breathing as well. Hear his heart beat as you lay your head on top of his chest. Know that you two have mated for life. Know that you two have now become one.