Dependency for parents, including in-laws, is currently assumed and need not be demonstrated, however, this will change for applications submitted from July 1st 2021, when dependency will need to be demonstrated.

The level of dependency that will be required is set out in the definition of “dependent parent”:
Having regard to their financial and social conditions, or health, the applicant cannot, or (as the case may be) for the relevant period could not, meet their essential living needs (in whole or in part) without the financial or other material support [from the sponsor].
Source: Changes to EU Settlement Scheme affect deadlines and family reunion | Free Movement

The following evidence is required:

a relevant document issued on the basis of that relationship, such as a residence card or a family permit (EEA FP); or
full birth certificate; or
other documents which satisfy the caseworker of the family relationship.

Parents, grandparents and great grandparents of the non EU spouse or civil partner of an EU national need to provide:
a marriage or civil partnership certificate recognised in the UK; or
a valid overseas registration document for a same sex relationship which is entitled to be treated as a civil partnership under the Civil Partnership Act 2004.

Dependency is assumed for parents, grandparents and great grandparents and it doesn't have to be demonstrated if the application is made before June 30th 2021.

Dependence is NOT assumed if the relevant EEA national is aged under 18.
If you have a valid PR or biometric residence card, see: Holders of valid PR and biometric residence cards

See also: Evidence for family members
Source: Evidence of relationship