There were no openly lesbian/bi sisters in my chapter when I was an active... but it wouldn't bother me at all. My sister is my sister, regardless of her sexual orientation.
Sadly, there was an incident with another NPC sorority at my school. A very popular and well-liked sister, who had held several offices and was very involved with her sorority, happened to be bi. She came out to her sisters at a sisterhood event (she was seriously involved with a woman at the time) ... and most of them promptly shunned her.
She was never again elected to office, many of her sisters began to avoid her completely, and she is not at all active as an alum or proud of her sorority membership because of the way she was treated.