I am an a-mom of twins. Their n-mom has an older daughter. We call her their sister. Not all brothers and sisters grow up with each other. Obveously they will probably not have the "brother/sister bond" that siblings share when raised under the same roof, but that does not negate the fact that they are in fact brother/sister.
This is only a small part of the complexities of adoption/ open adoption. But don't be affraid of it. :)

I think brother and sister is appropriate and shouldn't cause confusion in an open adoption.

Interesting question.  My children are half-siblings but we have adopted them all.  We do not refer to them as half-siblings because that would be akward, however true it might be.  Their birthmother has a fourth child and we just say, "birthmom's little girl".  We're allowing the children to make that familial connection on their own as they get older and are better able to process any emotions they might experience with it.  Because the children have different fathers, it is easily assumed that they have other half-siblings but we do not have connections with any of those men and it is not an issue we are faced with at the moment.  We are not intentionally being dismissive with the familial relationships, rather, knowing the emotional and mental abilities of our children, we are currently in a "discuss only when they bring it up" mode. (my oldest two children are emotionally and or mentally impaired)

While I am not anywhere near having my own family I do plan on having children one day...this topic actually came up the first time I met Cupcake's (my placed daughter's) Mom. She mentioned that she was probably not going to adopt again and asked if I planned on having more children. I told her absolutely that I hoped to one day get married and raise a family. She was so excited and said, "Oh great! So Cupcake will have brothers and sisters!"

While they won't live together like "traditional" siblings, that's still what they will be to each other in their own unique way.

I grew up with two half-sisters that I never lived with and rarely saw, but I still absolutely consider them my sisters. My full bio sis however, who DID live with me, doesn't consider our half sisters her sisters like I do...it's really whatever works for the family and the individual.

My daughter and son both have "half-"siblings by their birth parents.  My daughter's sister calls her (my daughter) her "half-ways sister." She was 4 when she coined the term.

Mainly we just refer to them as siblings, or sometimes "Noelle's children" when someone doesn't feel like explaining the whole complicated relationship.