A brief personal history: my parents have been divorced since I was two. I lived with my dad for most of my life, with frequent extended visits to my mother's place over the summer and holidays. Both of my parents remarried. My mother had two children with her second husband; my dad never had anymore kids, but he acquired a daughter - my older sister - through his marriage to my mom. So I have two younger siblings, but they've always lived too far away to really have much of a connection with them. I'm 7 years older than my brother and nearly 13 years older than my sister. I never had much of a connection with her, which is regrettable.
So now I've sort of acquired a sister through Elle. She has a sister two years younger than she who lives with her. Madison - Elle's sister - is a wonderful girl, but she's never had a boyfriend, and for some reason I have this protective streak running through me that makes me want to watch out for her. I suppose in a way it's just in my nature; on the other hand, I think that her being in my extended family makes me feel a bit more justified.
I think that keeping your significant other's (or others') family in mind is important. There are only a select few people in both my and Nyla's family who know about our lifestyle. There are even fewer in Elle's family, but those who do know are important to me because they're important to her. A relationship like this bring a lot of complications and stress, and a level of comfortability between family members seems to make things a lot easier on everyone.
One just has to remember to keep in mind that not everyone is going to be comfortable with your way of life and if there comes a time when they just don't want to deal with it - or you - that you have to deal with it and move on.