Hi Bucko!
"Kyoudai" can mean both "siblings (regardless of gender)" and "brothers" depending on context.
In most cases, it means siblings. Regarding Jim Breen's examples, maybe it was just coincidence?? I'm not sure...
When kyoudai is used with special attention to gender, it can be written 姉妹、兄妹、姉弟 as well, although 姉妹 can also be read "shimai."