While I love OI, one of the things that bothers me about a lot of series is how they tend to pit women against each other, especially in families. There's a lot of series with positive brother-sister interactions (though it could be argued they swing too far in the other direction), but a sister generally either doesn't exist or is a bitch.
Below are some series that portray supportive sisters along with some of my personal opinions. If you have any recommendations, please share.
Cassmire: The Loyal Sword
Half siblings. Little sister is a bit intense in a good way, but it makes sense considering she and the MC only had each other to rely on for a long time. I like this series so far. Not really a spoiler since it's in the first chapter, but marking it on principal. I love that the MC genuinely cares for her sister.
spoiler
She loved and took care of her long before she realized she was isekai'ed and once she found out, it was basically, "nothing changes except I can now send my sister to a better life even if it's not with me."
Cheerful Countess Sisters (Complete)
Half siblings. The first part of this series was phenomenal, but I hated it from when the romance started. It went from a ten to a two with the unnecessary drama. Worth reading for the first part where the sisters get to know each other.
The Princess in the Dumpster
Half siblings. It's been a long time since I read this one so not sure how it is to update. I love that she has an awesome older sister AND brothers AND all the siblings treat each other sincerely (the older sister is not pushed aside for the new younger one).
Everything's Coming Up Roses
Step-siblings. One sister and one brother. I dropped this because it got too boring for me, but it tops my list for healthiest handling of trauma.
spoiler
The dad hires a child psychologist understanding that he does not have the experience to help her heal on her own and she doesn't magically heal, the childhood abuse still affects her long down the line.
An Unseemly Lady
Full siblings. There's a large age gap between the MC and her older sister so her older sister also plays a motherly role. Very sweet relationship. Though be warned that
spoiler
it's a sad series because the author was realistic in portraying how people drift apart with time.
Though I May Be a Villainess, I'll Show You I Can Obtain Happiness! VOLUME 1 CHAPTER 2
Half-siblings. It's an anthology of short stories. Volume 1 Chapter 2 specifically has a story relevant to this list. It's a fast and satisfying read.
The Reason Why Raeliana Ended Up at the Duke's Masion
Full siblings. The sister only appears briefly at the beginning and middle so this series barely qualifies; however, it's one of my favorite series in general so I included it.
Two series I consider questionable for other reasons and don't recommend but technically include this dynamic with full siblings are My Sister Picked Up the Male Lead and I Became the Ugly Lady.
spoiler
(Yes, the series where her fat melts off)
For better or for worse has a good sister relationship