I wouldn’t be surprised if Lucy resents Elle, Lucy was like 6 when Elle was born, it’s obvious Lucy was close to her dad as they all spent together as a step family so it must have been strange for her when he remarried and had Elle and then katie obviously.
I know my older half sister was 7 when I was born, only difference being that she lived with my dad and not with her mum, but she hated me from day one, she literally bullied me so much my school thought I was being abused by someone because of how withdrawn I was!
not saying Lucy was like that at all, but she does come across as a bit odd and as people have said, she seems cold too.
also Lucy’s older sisters are twins so they are obviously closer to each other and with katie and Elle being full sisters they will have a strong relationship too, so it sort of leaves lucy randomly in the middle.