what are your ocs' relationships like with their siblings? are they close? prone to rivalry? or something else entirely?
Well, let's take it one by one, shall we?
Lothrin had two siblings. In her younger years they were inseparable, often doing their tasks as border guards together. Lainor, her older brother, died in the War of the Last Alliance and after the war ended in her grief Lothrin left Lorien and headed north. Aliar, her younger brother, never quite forgave her for "abandoning" their family in a time of mourning and ever since their relationship has been quite strained.
Astoeth is more often than not quite busy with her own pursuits, but she will always find time to spend with her older sister, Beleth who is a very accomplished blacksmith. Since I don't think the concept of "moving out of your parents house" exists in Middle-earth, they still live with their parents in their home in Celondim. They have their own shenanigans and sibling jokes. One thing they always do together is come home by dinner time in order to supper with their parents. Beleth also loves rowing a boat and will often ferry Astoeth on the river Lhun so that she may gather herbs on the more inaccessible banks of the river.
Melilot often worries about Adamanta and her bounder job, but her sister always tries to put her mind at ease. They have their sibling squabbles, that's true, but then what siblings do not? But they love each-other through and through. Adamanta wouldn't let Melilot leave the Shire on her little scholarly trips without her protecting her sister. Their bond was strengthened by the Scouring of the Shire.
Dōmēyondō (Dûion) loves Berékānis, but with him being away for such long periods of time, and being a messenger and all that, the two don't spend that much time together. Some siblings would rather not fight, but Berékānis loves riling Dōmēyondō up, and she always knows how since she's his sister. Some people really do love having friendly squabbles.
Sidhen used to spend a lot more time with her sister during their childhood. The two always got along quite well, but with Sidhen becoming a ranger and her sister marrying a scholar, settling down and having children, the two didn't keep in touch that much afterwards. Not that anything happened, it's just life you know?
Bron was brought up at a distance from her brother, since Hav was bound to be the next Brenin of their clan ever since his birth. It's quite hard developing deep bonds, when your sibling will one day be the one to rule an entire region and whose word carries the most weight around. And who will tell you what to do and what not to do. That's why Bron preferred the solitude of the wilds to the bustling atmosphere of the village. Bron hates being told what to do, and Hav in his sibling teasing wouldn't waste a chance annoying her.