I would likely not tell any of my siblings. Told all my parents, my auntie, and my cousins (well two of them).
I also think some people find it a threat. Is your sister married, herself? People can interpret like 'well, they want other partners, I don't want MY partner to know that other people are doing this; because my partner might think it's a good idea (and then my partner will find out I'm not good enough and leave me for the new one)(or, and then my partner will take what little love I'm getting and give it to someone else)(or all kinds of things!).'
I know when I'm exposed to new things, I wonder about what they would look like in my life. Sometimes it's interesting, sometimes it's meh, and sometimes it's downright frightening.
Love is that condition in which the happiness of another person is essential to your own...
Robert A. Heinlein
Me: female, bi, (formerly hinge of a vee)
with FirstBoyFriend (FBF)(moderately long-distance)
and no longer with CurrentBoyFriend (CBF)(who lives in the apartment building next door)