I think you're probably right, Bohne -- after all, in UK law even your own first cousin would be OK, and I suspect 2nd would be permitted for Catholics.
But aren't some other non-blood relations forbidden? Perhaps your sibling's widowed spouse, or your widowed step-parent?
legally they can, in UK law aslong as there is no blood relation you can marry who you like. but as far as i know, being drilled every day by my catholic mate, you cant marry a relative, blood or by marriage. by doing so you are marrying into a family that you are already related to and so are marrying a relative! complicated i know but thre you go!