Maine's highest court gave a legal victory Thursday to a woman who stands to stake a claim to a share of one of America's premier business fortunes thanks to her adoption by her lesbian partner.

In a unanimous decision, the Maine Supreme Judicial Court overturned a 2008 lower court decision that annulled the adoption.

At issue was whether it was legal for Olive Watson to adopt Patricia Spado in 1991 in Maine, where the longtime partners spent several weeks each summer on the island town of North Haven. Watson was the daughter of the late Thomas Watson Jr., who built International Business Machines Inc. into a computer giant.

The relationship ended a year after the adoption. Thomas Watson's heirs challenged the adoption in court in 2005.

Thursday's ruling is the final word on whether the adoption was legal, Spado's attorneys said. But it's up to a Connecticut court to determine if Spado is entitled to any of Thomas Watson's fortune.

"It means she is Olive Watson's child. She's a Watson family member," said Clifford Ruprecht, one of the attorneys who represented Spado in the Maine case.

Thomas Hanscom, a Rockland attorney who represents the trustees of the Watson family trusts in the adoption appeal, could not be reached for comment.

Olive Watson was 43 when she adopted Spado, who was a year older, as a way to protect her financially.

Watson's father was unaware of the adoption when he died in 1993, and the adoption wasn't an issue until after his wife died in 2004. With the death of both parents, the Watson grandchildren became eligible for cash payouts at age 35.

Spado's lawyer notified the trusts that Olive Watson's former lover was also a legal granddaughter and entitled to a share of the fortune. Trustees challenged that claim in probate court in Greenwich, Conn., where a judge ruled that Watson did not recognize Spado as his granddaughter and did not intend for her to benefit from the trusts.

Spado appealed, but the case was put on hold while the trustees went to court in Maine in an attempt to have the adoption annulled. To do so required evidence of fraud or deception.

In their petition seeking annulment, the trustees alleged that the couple obtained the adoption through fraudulent means by not disclosing their relationship to the court. The petition further alleged that Spado and Watson, as New York residents, had not fulfilled the statutory requirements of living in Maine at the time of the adoption.

A probate judge who granted the adoption in 1991 annulled it in April 2008 on the residency issue.

Spado's attorneys appealed to Maine's highest court, arguing that Spado complied with the requirements of Maine's adoption statute at the time. They claimed that even if Spado was not fully compliant with the statute, there was insufficient evidence that she committed fraud to justify an annulment.

The trustees also appealed the case, arguing that the adoption should have been annulled on the grounds that it was obtained by two partners seeking to manufacture inheritance rights who did not intend to establish a normal parent-child relationship.

In Thursday's decision, Maine's justices ruled that even if Spado did not live in Maine under the law, the adoption should not have been annulled because there wasn't enough evidence to support the claim that she had committed fraud.

The court also rejected the trustees' claim that the adoption should be annulled based on a public policy prohibiting adoptions involving same-sex couples. Historically, adult adoptions have been recognized as a means to convey inheritance rights, to formalize an existing parent-child relationship or to provide perpetual care to a disabled adult adoptee, the decision reads.

The case now will now move to a Connecticut superior court to determine if Spado is entitled to any of the Watson riches, said Michael Koskoff, her lead attorney in the Connecticut case.

"Now the fireworks will start," he said.

Olive Watson could not be reached for comment. Spado's attorneys in Maine said Spado would not be available for comment.