Here's one I wrote:

We've been best friends,
through thick and thin,
and I know everything you know,
because I love you.

You helped me through life,
you picked me up when I fell,
and all I can say,
is I love you.

People may think I'm crazy
that I can love this you this much,
but they don't know how much
you really mean to me.

I help you,
and you help me,
we shall always be together
because of love.

I'm here for you,
and I want you to know
words cannot express how much
I love you.

I will always have a shoulder to cry on for you,
I will always be here for you,
and I will always be your friend.
No matter what.

You're my sister,
my companion,
my teacher,
my leader,
my guide,
and my role model.
But the only thing that matters,
is that you're part of me.