Sorry, this is very long...
I am European, DH is African. We have been married a few years, love each other, no problems, baby on the way.
Both our families live overseas for the most part, with the exception of his sister who we see approximately monthly, very friendly but casual (odd restaurant dinner out together).
I get along with her casually, but keep getting the feeling she is trying to engage in some sort of power struggle with me whenever we get together. An example: insisting that the baby be given an African first name (he has father's African middle and last name, but our joint choice of European first name), and that it would be disrespectful to their culture otherwise. We stand firm and tend to deflect or distract the conversation.
My husband and I are very flexible with each other, and manage to come to agreeable terms on just about everything without any problem (so I'm normally not bothered by sister's comments), but our last meeting with her really shook me, and I'm not even really sure why as I'm sure I'm right.... or am I?
She asked us about would help with childcare after the baby is born. We have been considering paying my younger sister (free, single, would love the opportunity) to come spend a few months with us to help out, keep me company, and babysit when needed.
DH's sister said that this should be offered to their older sister (retired, single, would love the opportunity) first as she is older and according to their culture it would be disrespectful to not offer the opportunity to her first.
Without waiting for a response, she launched into a lecture in which she said I would have to learn these things if I expected to have a happy marriage, that I would have to respect their family's way of doing things, that she would not allow anyone to disrespect her older sister, etc. etc.
I replied that I understood our families had different ideas about the way things should be done, that I expected flexibility and understanding on both sides, and that the decision would be between me and DH.
She went on to say that she would not allow anyone to disrespect her family, and that her brother might marry 10 different women as far as she was concerned (I am his first/only wife), and he would still be her brother, and that the family's needs come first. 
I felt a bit angry and attacked, and didn't really respond except to say that DH and I would come to an agreement and I was sure there wouldn't be a problem. DH had kept silent but started getting his coat on, so I followed suit and we left.
In the car we were fine, DH reaffirmed that he thought my sister was a better choice as I was the one who would be at home more often, and we would enjoy each others' company. I didn't say anything about how I felt because I was still trying to figure it out myself (whether I was over-sensitive/hormonal, etc).
I know that essentially we will carry on doing things as we please, and as DH and I agree on, but can't help feeling really irked by the conversation with his sister and can't put it out of my mind. Is it possible I am really doing something ignorant/harmful to the family? Or being selfish? Or am I well within my rights (especially as DH agrees)?
Sorry for the looooong post, but this is really bothering me, and I can't even really work out why.