11) Is the following biconditional true? Explain. Two people are brother and sister if-and-only-if they are siblings.

Well, not exactly.

Step-by-step explanation: You see, there's this thing called half siblings. They aren't exactly quite any different to normal siblings, except the fact that they either have the same mother and different father, or the same father and different mother. This usually results after a mother and a father divorce, or their respected love one dies, and they have children. The mother/father then can remarry, and when their wife/husband also has a child. The two or more children become half siblings. Also, if we're gonna talk not literal siblings. Close friends often like to think of each other as siblings, though they aren't biologically siblings. Half siblings and step sisters are two different things, half is where your sibling shares half your genes, and step siblings are where they don't have any of your genes and only their respected parent's genes. Now depending on if they mean biologically siblings, I would say true. But if they count all, then it would be false.