Help with an Ask

Anonymous says: Hello, fellow cat lover here, I am in need of some advice, I have two cats who are siblings and recently one has been getting quite jealous of the other, he attacks her when we are holding or even petting her and she’s started show signs of stress (not using the litter box). This is my first time owning cats and I’m unsure of what to do, help?

Opening this up to suggestions/advice for Anon.