@so many questions ,
Please realize that this is an international site. It's after midnight here on the East coast. In England, its not quite 6 in the morning.
I wasn't going to answer but you should realize that there is a sexual orientation called bisexual. Not saying you're that. I don't know you to make that kind of judgement.
Quote:
noun
4. a person sexually responsive to both sexes; ambisexual.
http://dictionary.reference.com/browse/bisexual
Also, you may not actually be sexually aroused by your step-brother. I'm assuming you're at least a teenager or older but still in puberty. That you are going through certain hormonal changes at this time. It could be just a coincidence that you are physically reacting as a side-effect of all those hormones in your body at the same time your changing in the same room as your stepbrother.