Two Sydney teenaged sisters have launched legal action against their family to try and secure $90 million left to them by their grandfather who died leaving a $2.5 billion estate.

The sisters don' t want more money, but argue the Will doesn' t make "adequate provision" to ensure they receive their share as they are beneficiaries of two family trusts.

The legal challenge centres on concerns the value of the inheritance held in trust will diminish over time.

Wills expert Joshua Crowther of Stacks Law Firm said the case illustrates how people can claim against Wills, even if they are due to receive what most people would consider more than enough.

Mr Crowther said that under NSW law if a person leaves assets as part of their estate it can be challenged by certain people under Section 57 of the Succession Act 2006 (NSW).

"This automatically includes spouses and biological children but also includes stepchildren, carers and friends so long as they show they were members of the household and dependent on the deceased," Mr Crowther said.

"The latter category also need to prove what the law calls ' factors warranting' meaning they have to show that their relationship with the deceased was so close that the deceased had an obligation to provide for them in the estate."

But Mr Crowther, a Law Society Accredited Specialist in Wills and Estates, says this just allows them to commence a claim. Whether they are successful will depend on the strength of relationship, size of the estate, intentions of the testator (the person who wrote the Will) and most importantly – the financial need of the claimant.

"We' ve seen over the last 18 months that courts are less inclined to make provision for claimants, especially adult children, although this depends on subjective facets of each case.

"It can also depend on legal arguments put for the claimant and the sympathies of the judge hearing the case if it gets to court, where it is notoriously difficult to predict the outcome.

"Most disputes are settled in mediation, and it' s usually far better to resolve cases before they get to court. It is cheaper and should be a result both sides can live with."

Mr Crowther advised anybody thinking of challenging a Will to get expert legal advice before they embark on a legal challenge that can be costly without achieving the desired result.

"Challenging a Will can be based on a very emotional response, and it' s vital people get expert independent legal advice and not let emotions drive the case."

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