Science versus society
Joking aside, there are good reasons why finding love with a biological relative tends to be more than a little frowned upon. From a purely scientific point of view, first cousins are more likely to have children with congenital genetic disorders, for the simple reason that any weaknesses in the gene pool are more likely to appear when both parents' share that genetic trait themselves. Last year, in fact, Labour MP Ann Cryer called for an end to marriage between first cousins after statistics revealed that British Pakistanis - for whom such a practice is normal - are 13 times more likely than the general population to have children born with genetic disorders.
But that doesn't explain why sexual relationships between stepbrothers and sisters - who, naturally, share no genetic link - tend also to be viewed as warily taboo. No, I suspect the reason we're equally fascinated and repelled by the idea of love among family (biological or otherwise) is because a line has been crossed. The so-called innocent love of family members who are perceived to have grown up closely together, trading video games and insults as only relatives do, is somehow tarnished by sex.
A family affair
Despite the fact that they are not in any way related, a sexual relationship between a stepbrother or sister - let alone genetic cousins - can feel a little too much like incest to some of those observing from the outside. Never mind that people meet in all kinds of weird and wonderful circumstances these days, finding romance via your parents' romance or with someone you once shared a sandpit with, has the ability to shock our modern morality to the core.
Even leaving the sex aside, people continue to be fascinated by such couplings for the simple reason that they tip all the normal rules of family life firmly on the head. I know this first-hand because, half a dozen years ago, my sister married our first cousin. Now, my sister's adopted so there was no bloodline to worry about. Likewise, our families grew up having almost nothing to do with one another so my sister and our cousin met as adults who, as adults, experienced a very instant attraction to one another.
Even so, since they got together nearly a decade ago I've encountered endless bemusement and fascination from anyone who learns about their familial as well as marital connection. And I can see why: to outsiders there is something peculiarly curious about having a brother-in-law who is also my cousin, while my mother's even more convoluted relations (mother-in-law and aunt, sister-in-law and sister, and now both grandmother and great aunt) are enough to make your head spin. However, as is always the way with these things, they only tend to look peculiar from the outside; in the eye of the familial storm all is, for the most part, pretty calm and serene.
Getting to know you
Which isn't always the case for those in similar circumstances. If your love fails to be reciprocated, for example, the proximity of an often-seen cousin or a live-in stepbrother can be excruciating. What you have to realise is that in this, like any other romantic situation, you can't force another person to love you. You need to do what you would do in any similar situation - give yourself space away from that person so that you can work out what's really going on in your head.
Falling for someone close to other members of your family potentially has consequences beyond the parameters of your own relationship - any ructions or fallings out will most likely resonate through your wider family circle and might pressure those who would otherwise choose to remain neutral about your love life to feel that they have to choose sides. My sister and her partner took three years since that first, initial attraction to act upon their feelings. That time allowed them to both clarify their feelings and trust them to be true.
Approached sensitively and with candour there's nothing inherently wrong with falling for a cousin or stepbrother. Just remember to tread that little bit extra carefully. In such a situation the stakes are slightly higher - it's worth taking the time to remember that there could be a lot more for you both to lose.