I found part 2 waaaaay easier than part 1.

literally even though its a 3v1 they take turns. simply backstab and or combo them while staying on the move to avoid projectiles.

then quickly dispatch the last one before big brother comes out.
big brother has the exact same easy to improv dodge/parry as his first fight AND you can use the pillars and bags nearby to bodyblock his attacks for safety and to cheese him.

part 1 is easily twice as hard because they gank more aggressively, you have less levels and weapon options, and less armor, less rings. less quartz etc

They don't fight you fair, so you don't have to fight them fair. They're only obnoxious if you fight them up front and they aren't very effective at chasing you. Naturally, the tiny bits of terrain you can use help a lot.
Falcon Eyes, Aegis, and the DLC arms Cataclysm and Icarus are all very powerful against them and going for backstabs wherever possible is always rewarding since it guarantees you can't get grappled from offscreen or pelted with a throwing cell.
If you take out the last two rabbits quickly enough, then you can focus on the eldest without any interference whatsoever. It's just a matter of wearing them down at the end of the day.

The first Brotherhood fight isn't too bad either, since the harassing rabbits only spawn once you've depleted enough of the eldest's health and you never have to fight more than two rabbits at once. The eldest similarly leaves you alone if you keep enough distance, shouting his very telegraphed running attack otherwise.

Originally posted by Bubblehead:

And it's Black Rabbit Brotherhood pt 2. ll the tips I read is to focus on the one that's buffed as that's the primary attacker and no one else will attack you. That's b.s. You have to be out of their range, but then they randomly throw ranged attacks at you. You take two hits and you have a status effect. I ♥♥♥♥♥♥♥ hate this fight. The embarrassing part is this is my 5th playthrough. I beat them legit my first playthrough back when it came out so I know I can do it. But my other playthroughs, I used that amulet that blocks all status'

The AI is stupid, idk what weapon you use, I used the rapier, and I just spammed R2,
Poke
Jump back
NPC comes running at me
Poke
Jump back
NPC comes running at me
rinse and repeat

Most of them will follow you relentlessly if they're in attack mode, I hope this knowledge helps you somewhow.

use the mine legion (deus ex machina), use human damage amulet, play around the pillar to split them up, dodge forward and left if you have issues deflecting, end the fight with the elder by throwing stuff at him

One method i used before the helper was buffed, as he would die super early in fight so ended up going solo anyway, was to drop one sibling fast, then fight the other 2 so they would die soon, so when the elder woke up, the last sibling would drop leaving me with a 1 on 1 with the eldest.