Why it’s not uncommon to fall in love with the partner of a late sibling.

Just this week, former US Vice-President Joe Biden found his family life, not his politics, the centre of international news.

His son, Hunter, had spoken to Page Six of a new love and new relationship after separating with his wife, Kathleen, in October 2015. His new partner was named as Hallie Biden, widow of Hunter’s brother, Beau, and former sister-in-law of Hunter himself.

“Hallie and I are incredibly lucky to have found the love and support we have for each other in such a difficult time, and that’s been obvious to the people who love us most,” Hunter told the website.”We’ve been so lucky to have family and friends who have supported us every step of the way.”

Beau Biden died in May 2015 of brain cancer, leaving behind Hallie and their two children: a daughter, Natalie and a son, also named Hunter.

Generally, the news was met with well wishes and positive sentiments were aplenty. After all, grief and life are fickle things. Love and companionship; beautiful things. And when the two intersect? It’s complicated, like grief and love almost always are.

It’s almost impossible for us to comprehend the emotions involved in grieving the loss of a husband while falling for his brother. It’s simply a complexity too great, so most have bid them good luck, and hoped happiness finds all involved.

But for some, the question still lingers. How?

It’s not unfair, or uncommon, for many to wonder how that kind of relationship is born. It’s human nature to inject yourself into that same scenario and hypothesise if you could, or would, make the same choice.

And although questions have been raised and eyebrows furrowed, the most interesting notion of all is that some would. Some would make the same choice, because it's much less about choice and much more about circumstance.

Refinery 29 understood the natural confusion, and spoke to those who understand human emotion best to delve into the idea that falling for your family member's widow isn't totally unheard of. In fact, Kristen Zeising, a couples and sex therapist, told the website falling for your late sibling's partner isn't uncommon at all.

"When you go through a traumatic experience or a highly emotional experience, that can be a bonding experience," she says.

According to the Association for Psychological Science, despite its unpleasantness, pain may actually have positive social consequences for bringing people together.

“Our findings show that pain is a particularly powerful ingredient in producing bonding and cooperation between those who share painful experiences,” psychological scientist and lead researcher Brock Bastian found.

Lena Aburdene Derhally, a licensed psychotherapist who specializes in grief concurred with these ides, telling Refinery 29, "their grief was probably matched in certain ways and in aspects that most other people would never be able to understand."

As such, the relationship becomes much less about genetic overlap and much more about sharing an impenetrable bond. There isn't an emotion much deeper than a prolonged state of grief, and it makes sense to gravitate to the people who understand that feeling more than anyone else. Grief is a lonely place.

In a more historical sense people marrying their sibling's widow or widower goes back to biblical days, where second marriages stayed within families to share resources and provide security.

In modern times, though, according Dr. Zeising, it often comes down to a widow, or widower's, inherent desire for companionship. And it makes practical and emotional sense for that companion to share, and understand, your grief.

"Some have a hard time being alone if you've been with someone for so long — you want a partner.

"But you certainly don't want to find just anybody to fill the space," she told Refinery 29.

Perhaps for the confused, it goes a little of the way to break down the how. It's not a freak show - it's pure human emotion.