Iām the result of my mums second marriage, and my older siblings are from my mums previous marriage (though I consider them to be my proper full siblings!). My mum did a great job with it, and despite her ex husband cheating, they were really amicable etc (weād sometimes have him at Christmas so my older siblings would have all their family together.)
BUT- my mum DOES treat us slightly differently, which she can somewhat see- she doesnāt have to ācompeteā for me and my full sister. Sheās had to ācompeteā for my older siblings who would go between living with us and their dad (made harder because he was well off and took them on lots of holidays etc). Sheās aware of this and tries to not let it affect how she treats us etc, but she does sometimes naturally default to letting the older siblings get what they want, even when it gets in the way of mine and my sisters needs.
I think Louise probably feels like Pearl is āhersā, easy, sheās always there and (presumably) always will be. No need to worry! Focus on Darcy! Make sure she wants to be here and not at her dadās! Make sure she prefers me to her stepmum! Itās unhealthy all round