Struggling with my younger sister's arrogance and cruelty

Assalamualaikum, I need some advice. My youngest sister and I are 4 years apart. Growing up, I saw her as my best friend, but she often seemed jealous of my school achievements. My parents always doted on her, I think to make up for her academic struggles. Honestly, I'd trade those achievements for their love-I felt I had to earn their validation through grades. After she went to poly and uni, found a close circle (especially after meeting her husband through an app, and a friend became a doctor-she'd mention her to our mom), she became arrogant and rude, especially towards me. We haven't spoken in 2 years. She treats me like I'm invisible. When I bring food for the family, she never thanks me, but she does for my other sisters, loudly. I've asked my mom to help, but she says my sister is too egoistic and hard-headed to change, and she's scared to upset her in case she goes berserk. My father is emotionally absent and now mostly bedridden. Recently, we shared a room at our eldest sister's house. I paid for a wall cabinet for storage. I moved out recently (thankfully, as her late-night gaming and calls with her husband disturbed my sleep) but left some electronic boxes. When I needed a box for warranty, I asked in the family chat because I noticed my stuff was gone-the cabinet was filled with her things, without permission. Then she sent rude comments, like: "At least I wasn't unemployed for almost a year after graduation :)" (that was 10 years ago, and I worked part-time while job hunting), "At least I got a job in 2 months, jokes on u, my pay is higher" (she got her job through a friend's referral after being let go). She also said, "Turning 30 but don't know how to act like one, what a loser" (she didn't even wish me on my birthday). And, "My life has been so much better without talking to you." I'm livid. How do you deal with such a person? My mom privately asks me to forgive her. On the surface, her life seems perfect: she's engaged to a national athlete (who's my age and earns less-not sure if she looks down on him too), and her father-in-law is an imam. She acts pious outwardly, but we don't see her recite Quran at home. She's giving prayer mats as wedding favors; my mom wanted to add plates for guests of other faiths, but she snapped, "This is my wedding, I decide." It's hypocritical-she's nice to friends and travels a lot, but she's so mean to family. My parents are letting her and her husband stay in their flat (which they rented out for income) until their new home is ready in 3 years, while they continue living with my eldest sister, saying they need privacy. Life's been tough as the black sheep with little support. I'm losing sleep and even took leave from work feeling light-headed. Any advice on how to move on? JazakAllah khair.