The definition of children includes nuptial, ex-nuptial and adopted children. Step children and foster children are not considered a “child” of the deceased in the context of a Will and therefore unless they are specifically named as a beneficiary in the Will they would not receive any part of the estate.
The reference to brother(s) or sister(s) in a Will generally includes half brothers and half sisters, however to be safe you should name all beneficiaries.