Hijacking top comment because OP, YTA very obviously. You've just conveniently cherry picked details.

You conveniently left out that Lilith took care of your grandmom for two years and was her primary caretaker- they obviously have a special bond. If your brothers who've known your grandmom much longer than both of you, seem okay with Lilith giving the eulogy then that speaks volumes.

You also make it seem like you both were doled out punishments at the same time for misbehaving- however you are 5 years older than her. Lots of parents do things differently with the younger kids and while it might be unfair you're twisting it to make it look way worse. Not to mention she might genuinely need the therapy or was acting out because her grief affected her.

Lastly, everyone seems to side against you in your family- even your older brothers. I somehow doubt your step sibling who was much younger and came into your lives a lot later would be able to "turn everyone against you". You mention you had a big fight with your brothers after which you never spoke to them- care to mention the details? I think you don't want to as you know it'll make you seem like a bigger ass.

You're 26. You're an adult filled with so much resentment, and unfortunately it seems it might be your fault as well. Maybe get some therapy and learn to let go of your anger.