My girlfriend's sister, who I have know for years, has invited (only) me to the birth of her first child for support. That is her free choice. I don't know who the father is and I don't ask. I will probably be too embarrassed to even look at her 'below', but I may do. However, I will tend to stay focused on holding her hand, offering her words of support in a quiet way, and have already started to look at sites about what the role of a birth-partner is.
My girlfriend (her sister) thinks it inappropriate, but has yet to provide me with convincing reasons as to why it is inappropriate, but I know her sister would never have another woman with her (we have discussed it several times) and her dad is not in the area. I also know if my girlfriend had been invited it would NOT be considered inappropriate. Apparently, if I attend it is voyeuristic and sexual, but if anyone else does, it's not, and that is the hurtful part. It's not my baby, but myself and the sister simply get on well. I am a kind of neutral in her family and don't get involved in any family arguments - always keeping my distance - so I am assuming she sees me as reliable which is probably what you want on the day in question.
Why would her inviting me cause such a rumpus - again, she would rather have nobody there than another woman: I am just amazed at the double-standards of some people.