What is Whitley and Weiss's relationship like? Is there resentment on both their ends?
Whitley and Weiss are two Golden Children in different parts of its life. Weiss was the original, the heiress and progedy that Jacques puts all the attention and expectations on because he saw her as the future. Whitley, on the other hand, was pretty much the spare and left in the background, watching Weiss get everything that he wanted.
And that environment brewed competition and strained whatever relationship either of them could have. Whitley was driven by jealousy and a desire for affection, while Weiss simply saw him as someone in her shadow, but also someone who could very much step up if she faltered.
And that was what precisely happened when she chose to go to Beacon. In taking that one decision for herself, she opened a position that Whitley was all but eager to take advantage of. It led to her falling from Jacques' grace and Whitley being held up instead, that competition fueled by their father as he constantly held Weiss' decision and Whitley's achievements over her heads.
But he would also remind Whitley that his position at that moment was just as fragile as it was for Weiss. He knew that if he messed up, he would be at the bottom again.
So now the positions are flipped. Whitley is the prodegy while Weiss is the nuisance, and this gets even worse when she comes home and the possibility of his fall becomes more and more real. Her presence there is a constant reminder to him, even if Weiss is now in the mindset of not wanting to follow in her father's footsteps anymore.
There's also the same reason he hates Winter as mentioned in this ask. Winter came back for Weiss; trained her, guided her, but she did none of that for Whitley. So while he hates Winter for abandoning him, he hates Weiss for always being the one in the spotlight. The one everyone loves and dotes on while he was stuck in the background.
When she left for Beacon, it was a moment of relief that gave way to fear because now there was no barrier from Jacques' abuse. Deep down, Whitley does know that she had to deal with the same thing he does now, but it's twisted into blame because she left him to pick up the mess.
And Weiss at the point where she comes home after the Fall also does not like Whitley. While she doesn't see him in the exact same light as before, him copying Jacques' behaviours and thoughts puts her on edge and makes him untrustworthy to her, something that is even more sad because Whitley is only doing it to survive.
Thankfully, their relationship gets better when she comes back to Atlas with her team and they finally take care of Jacques.