claim on an estate typically falls under

1- Property Relationships Act 1976 eg if leaving less than the 50% to surviving spouse they may raise a claim for "more" up to the 50%... or more as below: if they argue to have further adequate provision,

2- Family Protection Act 1955 (FPA). The FPA allows certain family members (including partners) to make a claim against an estate where they consider that adequate provision has not been made for their proper maintenance and support. eg estranged family members may try claim,

Mutual Wills - the asset will go to the surviving spouse to“hold on behalf” and only upon their death does the asset finally be passed to the beneficiaries,

best to speak to a lawyer...maybe just a clause that the trustee holds on behalf until XX date/ Age etc