She saw a penis and scrotum where her pussy used to be.
Steve walked behind the three of them and watched as one of them would drop his hand from Sue's waist to feel her bottom. Once in a while Sue would reach behind her to tap the hand away but she was definitely enjoying the attention.
Sue's chest had been gleaming with sweat from her dancing and although she had slipped her jacket over her shoulders the two taller men were able to peak down at cleavage and her still very erect nipples. When the group walked into the chalet Steve put some music on while Sue made some drinks. When she returned with the drinks the two men stood up to accept them and thank her.
Steve went to collect his and Sue's drinks and when he turned around the guys were both leaning down and kissing Sue's hands. Steve felt strange as he watched both men began to work their way up Sue's arms, placing gentle kisses on her bare skin, but he said nothing. He convinced himself that nothing untoward was happening by remembering that Sue had kissed men at parties, either during party games or for Christmas or New Year, etc. But then it occurred to him that the kissing had not been like this. The young men had made their way to Sue's shoulders and were slowly converging on her neck. Sue was standing with her eyes closed and her head tilted slightly backwards, inviting kisses on her neck.
When they reached her neck Ian moved so that he was kissing the back of her neck whilst Dan kissed the front. At almost the same time Steve saw one of Ian's hands stroking Sue's back and bottom whilst one of Dan's was stroking her stomach, briefly passing over her skirt and moving up towards her breasts. Steve was frozen in place watching the seduction progress. There was very little doubt that he was enjoying it almost as much as Sue seemed to be.
Dan's kisses moved up from Sue's neck and his mouth eventually closed over hers. Steve heard a slight moan from Sue and then the couple started French kissing very passionately, with their tongues darting in and out of each others' mouths.
The kissing stopped and Dan took hold of the bottom of Sue's t-shirt and started lifting it up her body. Sue's eyes opened and sought Steve's. He nodded encouragement so she raised her arms to allow the t-shirt to be removed, revealing her bare breasts to Steve's and Dan's gaze. Steve was sure that he'd never seen her nipples as large as they appeared now. Dan didn't hesitate. As soon as Sue's breasts were freed he held them in his hands and re-started French-kissing her.
Both of Ian's hands were resting on Sue's hips, then they moved down her thighs and disappeared under her skirt. As they moved back upwards, Sue's skirt was gradually raised, revealing her stocking tops. Sue was taken over by the pleasure she was feeling and when one of Ian's hands moved back down and across to try to access between her legs Sue moved her feet apart to allow access to her panty covered pussy. Steve's mouth felt dry but there was no way he was going to interrupt the proceedings.
Steve saw Ian's fingers appear at the front of Sue's legs, with the palm of his hand resting against her slit. Then they moved backwards and stroked along the edge of Sue's panties. Sue started moaning louder and moving her hips as the touching continued. Ian's fingers deftly moved Sue's panties aside and started caressing her pussy lips, then two slipped inside her slit. Steve was amazed that Sue would so readily be French kissing one young man whilst being fingered by another. In response to the fingering Sue was moving her hips to force herself onto the fingers. Steve then watched as her hand reached down and started feeling the crotch of Dan, the brother she was so wantonly kissing.
Steve knew he ought to say something before things really went too far, but he just stood there sort of dumbfounded.