Gemma stood there stroking her dick, wondering how to proceed with events when suddenly it hit her "I've never actually tested this thing out!" She thought. She walked up to James and began to stroke his chest, teasing him. "Oh brother dearest..." She cooed seductively. "Bend over." James was frightened. "What? You wanna stick it in there? I can't do that!" He spurted out frantically. "Oh don't be a party pooper, you'll love it, I know I did. Just trust me." She whispered in his ear. "Now don't make me repeat myself! On your knees!" This time she was more forceful, pushing him down to the ground. He obeyed, but was shaking with fear.
Gemma knelled down and put her face in her brother's ass crack, licking and teasing his anus, well all the while applying lubrication. His body convulsed and moaned, unused to the alien sensation. Gemma got a little more frisky as time went on flicking her tongue in and out of his sphincter. Deciding that was enough she removed her tongue, a large trail of saliva dangled from James' ass. "Alright James. Here I come." Gemma guided her cum soaked penis to James' waiting anus, placing the tip perfectly at its opening. James could feel it now. She then began to apply pressure, her shaft slowly disappearing into the depths of his ass.
James grunted and groaned "Ugh. It hurts!" He complained. "Stop fighting it! Just relax and let it happen, you'll loosen up and begin to enjoy it in no time." Gemma replied. "I'm almost all the way in! Good boy!" Gemma buried the rest of her cock deep into her brothers ass her pelvis tight up against his ass. Both siblings let out load moans. Gemma now started to slowly pull back causing James to cum abruptly. "Did you just cum you naughty boy!? You like this more than your letting on don't you!" "N-No!" James stammered back unconvincingly. Gemma smiled "Well, I guess I'll just stop then. Is that what you want?" "No! Don't stop! Please keep fucking my ass sister! It feels so good!"
On hearing James' plea Gemma decided to turn things up into overdrive. She grabbed his ass with two hands and increased her speed, slamming his ass against the base of her penis each time. Gemma's breasts bounced up and down in rhythm with the slapping of their bodies as sweat glistened off her body. With each thrust Gemma felt herself closer and closer to orgasm. James was crying out in pleasure, having recently cum a second time. His body was undergoing rapid changes. His penis had reached it's maximum length clocking it at about 11 inches. Light tufts of fur had sprouted across his body and his feet had fused together becoming hooves. His mouth was now filled with large buck teeth and his ear's had spring up. He brayed loudly as he came for the third time in a row.
Gemma on the other hand had changes considerably, while fucking her brother her entire lower half had completed it's transition. She was balancing herself on James' ass, luckily her human hands were still capable of gripping it. Suddenly James' had his third orgasm braying in delight. "Lucky basted" She though. But then she could feel her own impending orgasm too. "Oh god here it comes James!" She screamed. She thrust in and out of his body until she felt the flood gates burst, the she pulled her younger brother all the way down her dick and flooded his bowels with her thick incestuous donkey sperm. It was a torrent, it kept coming and coming filling James completely, then sending her dick shooting out and spraying the remains all over his ass.
Both James and Gemma collapsed in pleasure breathing heavily. Their transformation's nearly completed. Gemma stood up first, or at least tried to. She found she could no longer walk on two feet, her hind legs taking most of the weight while she supported herself with her hand up the front. He breasts swayed below. "All right James time to get up. It's your turn now! So come and fuck me silly! complete our transformations! Turn us both into donkeys! James stood up, erection in hand, happily obliging his sister's request.