My two brothers are jerks. Just because we had the same parents, doesn't mean there is any connection that's meaningful.

I never had a sister, but always wanted one. Over time, starting long before transition, I have always adopted certain friends as sisters. I consider one or two women here to be my sisters. If anyone doesn't like that, tuff titty in the city, baby.

My sister is my sister and my brother is my brother.  I think they both still think of me as their brother, although my sister is - I think - starting to think of me as her sister.

I don't usually use the terms for other people, other than to show agreement - "Say it, sister!" or "Go for it, brother!".  It's not that the terms are reserved for people of special status, it's just that I don't usually speak that way.

I have a friend I grew up with who was more of a sister to me than my blood sister.  She said that I am no longer her almost-brother but am now her almost-sister.

My daughter calls me Maddie so that she can use female pronouns easily.  We both know I will always be her parent.  She introduces me to others as her mom, even when most of them know she has another mom.  (But some people do have two moms.)

We are all sisters and brothers in some sense, just because we are all human.  But I usually don't use those terms to describe my cohorts who have struggled as I have.  Perhaps it is because we struggle separately, wrestling with our own demons, trying to become our indivdual selves.

- Kate

I think I'm probably closest to K8 on this one.  I don't generally use the term for other than use in showing agreement, or on very infrequent occasions otherwise.
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I'm not particularly against it, and would accept the honorific politely if someone used it toward me, but it isn't one that I frequently use myself.