"Il a une sœur et un demi-frère."
Translation:He has a sister and a half-brother.
If you have a step-sibling, your parents are married, but your not related. It's a different concept.
Are there different ways of saying half-brother (ie shares one biological parent)and step-brother (ie a parent's partner's children, no biological relationship) in French, just out of interest?
According to my Cassell's F/E dictionary, step-brother = le beau-frère. Also: stepsister = la belle-soeur, stepson = le beau-fils, stepchild = le beau-fils, stepdaughter = la belle fille, stepfather = le beau-père and stepmother = la belle-mère.
Does "le bel-enfant" exist? (Since you wrote the translation for stepchild is le beau-fils)
No, 'le beau-fils' is used for 'stepchild' as well as for 'stepson'. Plural: 'beaux-fils'. Interestingly: the French also use the beau-/belle- prefix for in-laws, so, depending on context, 'ma belle-mère' can also mean 'my mother-in-law', 'a brother-in-law' would be 'un beau-frère', 'sisters-in-law' = 'les belles-soeurs', etc. EDIT: Yes, you can use either 'le bel-enfant' or 'le beau-fils' for 'stepchild'. http://fr.wiktionary.org/wiki/bel-enfant. Sorry about that, MThoriqMalano.
What about stepmother? Is it "la belle-mère", just like "mother-in-law"?
Here are some links that you may find useful: http://www.wordreference.com/enfr/