On behalf of everyone on this forum, let me offer you condolences on your loss. I have two sisters and I really can't imagine life without them.
"The Goblin Market" by Christina Rossetti is about sisters and what they will do for each other in an emergency. (This poem is very special to me and my sisters.) You can read the whole poem at
[
users.crocker.com]
The last few lines go:
"For there is no friend like a sister,
In calm or stormy weather,
To cheer one on the tedious way,
To fetch one if one goes astray,
To lift one if one totters down,
To strengthen whilst one stands."
==========
You can go to www.bartleby.com, choose VERSE, and search there for "sister" and "sisters" -- perhaps one of those poems will be useful for you.
YOU MIGHT ALSO CONSIDER using a poem that isn't about sisters per se, but about something your sister loved--animals, reading, plants ... anything that you think of in connection with her.