Certainly, you can date anyone you want, including your cousin. But your cousin's step-brother is not related to you, so questions of relationship do not arise.
It's about equivalent to dating your cousin, so it's up to you if that's against your morals. As for genetics, having a child with him won't produce abnomalities if they don't share a biological parent with your cousin, but as for ethics, it's up to you.
If the parents of your cousins step-brother are related to you no. But if they're not go for it!
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