This part was up to Silvyr. While Jack could have shouldered Aaron's weight, she was sure Silvyr, with is broader shoulders, would be better suited. It would leave her to handle the escape, which she had thought out. She had her plan A, and B, and then her scary, only-if-absolutely-necessary plan C. Her goal was to avoid plan C, but if she had to, she had to. So, she would leave Silvyr to get Aaron to the point where she could take over.
She sprung into action. The second Silvyr reached her, Jack whipped around, immediately sending patronuses down every corridor, to attempt to drive any dementors away. She began running, lighting the unlit torches to try and drive warmth into the forsaken place. Her eyes keenly searched each approaching corridor, making the turns that would drive them to the outer walls. She could hear the whoosh of dementors behind her. One of her larger patronuses arrived, attempting to run in between her party and the dementors, but it was beginning to fade.
She hated Plan C. But they didn't have a choice.
She pointed her wand ahead of her, at the side of a wall. "BOMBARDA MAXIMA! ACCIO BROOMS!" A large hole burst into the wall as rubble rained off of the building. Jack reached back and grabbed Silvyr's arm, and then turned, diving out of the building.
There was that surreal moment of falling, where it seemed like the brooms would not come. But Jack's broom was quick, and sped underneath her. She held to Silvyr and pulled him and his precious cargo onto the broom with her. It caught their fall briefly, but the weight caused the broom to sink and slow. The other broom pulled up beside them and Jack yelled over the roaring wind to Silvyr. "I'LL TAKE AARON! GET AS FAR AS YOU CAN, THEN APPARATE!"