Lots of societies practice incest (the ancient egyptians are some of the most famous with brother/sister marriages) to "keep the line pure". Well, we don't breed like horses or dogs, our recessives breed true instead of reinforcing dominants, in those types of relationships and "fresh blood" has to be brought in.
In many other socieities plural marriage between twins and 1 or 2 other spouses (so the twins wouldn't be separated) were VERY common, although by no means inisisted upon. In my readings, sexual relations seemed to be like a 'normal' threesome or foursome... meaning all 3 or 4 participate OR 'separate but equal' types where only 2 people are having sex together.
So could it be true?
Sure. There's quite a lot of historical and cultural precedent.
((FAIR WARNING: According to the Kinseys, and other child psych research... the most common 'first partner' for sexual experimentation is with siblings. Siblings touching tongues, french kissing, genital play, mututal masturbation, even penetration. Typically... this is all done before puberty... the most common age ranges for it are 3-6 and 8-10. THIS IS COMPLETELY NORMAL. The vast majority of siblings experiment to greater or lesser degree. THIS IS COMPLETELY DIFFERENT than molestation, or the romantic relationship described in the letter. Most of the time parents catch them, tell them to knock it off four or five times -like most things few kids listen after one 'Hey! Stop that!'-, and they do. Most kids don't even remember any of it because memories for so many people start at kindergarten.))