Joe Jonas and Sophie Turner's Transatlantic Custody Battle 'Going to Be a Thorny Case,' Legal Expert Says

Jonas filed from divorce from Turner on Sept. 5, and on Thursday the 'Game of Thrones' actress sued the singer for the return of their children to her native England

In the wake of their split, Sophie Turner and Joe Jonas are divided on where their two young daughters should reside — and lawyers tell PEOPLE that things can get contentious when it comes to international custody disputes.

On Thursday, Turner, 27, filed a lawsuit in the Southern District of New York requesting that her two children with Jonas — Willa, 3, and their 14-month-old daughter who has been identified in court documents as D. — be returned to "their habitual residence" of England. Per documents obtained by PEOPLE, the Game of Thrones alum’s legal team is calling for the return of the children to England, who Turner alleges has been wrongfully retained beginning "on or about September 20, 2023."

In the documents filed through the Hague Convention, an international treaty, Turner claims she and Jonas had agreed England would be their “forever home” after they sold their house in Miami in April. In recent months, the actress has been filming the miniseries Joan in England, while the couple’s daughters accompanied Jonas on tour.

Jonas, who filed for divorce from Turner on Sept. 5, responded to Turner's suit in a statement to PEOPLE later Thursday, saying when he and Turner met up in N.Y.C. last weekend, he said that he believed the former couple had "reached an understanding that they would work together towards an amicable co-parenting setup."

Now, Turner alleges in the documents that Jonas is withholding their daughters’ passports to prevent her from returning to the U.K. with their daughters.

Related: Sophie Turner and Taylor Swift Were 'Completely Normal' at Dinner 2 Days Before Custody Lawsuit: Source (Exclusive)

In the wake of the back-and-fourth, PEOPLE spoke with Marilyn Chinitz, matrimonial partner at Blank Rome LLP and David Glass, a California-based family law attorney, neither of whom are involved in Turner and Jonas' legal dispute.

"It's going to be a very thorny case," predicts Chinitz, noting that the court must primarily establish where the children were most recently established - their "home state." But due to the former couple's peripatetic lifestyle, this will be particularly hard to gauge.

"It doesn't matter where they spent the majority of their time," says Glass, citing the Hague Convention's clauses built around child abduction from one parent. "The question is: where did they spend the last six months or where did they spend the longest extended period of time in this case?" Typically, he says, "you need to live in a new international jurisdiction for six months before the court can say that that country is the children's habitual residence."

Jonas has claimed the children had been living in Miami and other locations in the U.S., while Turner said five months ago the family relocated in April to a London short-term rental and began house-hunting.

The recent sale of their Miami home makes it all the more complicated, Chinitz says.

"It's going to come down to, where are the roots that were established for these kids?" she says. "The reality is, they had a home in Miami and they traveled (a lot). So the court's going to say, but where did the kids stay? Where did they go to their doctors? Where did they go to their activities? What were the roots that you all established?"

In her filing, Turner says the children "are both fully involved and integrated in all aspects of daily and cultural life in England," further making the case for the U.K. being the children's legal home by citing their eldest's "full time" nursery attendance and local doctors.

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“The big argument is going to be, 'Did they ever have an agreement between them that they would make London their home?' " Glass says, noting Turner's claim in her Thursday filing that they entered into a contract to purchase a property in the U.K. in July.

"The parties’ shared plan was to purchase the property together and raise their children together in the beautiful home and English country surroundings," says Turner's filing.

But legally speaking "it's not enough to have a contract to purchase," says Chinitz. "You didn't close on that. It's not enough that you went to visit." To prove their shared intent, she says, brokers they toured homes with may be called to testify.

Further complicating matters, by offloading their Miami home, "they sold the one piece of property that they owned together in the United States," notes Glass. By not owning a piece of property stateside, he says, it may be challenging for Jonas to claim Florida as the children's primary residence.

Related: Sophie Turner Alleges She Learned of Joe Jonas' Plan to Divorce 'Through the Media'

Either way, "there's no slam dunk here," says Chinitz, as both lawyers say the children's citizenship between the U.S. and U.K. does not impact the judge's decision.

If a judge cannot establish a home state the question turns to, "what's the best interest of the children? Who should they be with? And that's going to be really tough because both of them have demanding careers where they travel, where they're away for extended periods of time," says Chinitz.

The couple's best bet would be to reach a private agreement via mediation or a private judge, both lawyers advise. "It's a case that ought to settle with two adult parents talking to each other. I think it's most important that they do, because otherwise one parent is going to be really unhappy," she says.

A hearing date for the federal case over where the couple’s children will reside has not yet been set.

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