https://www.iras.gov.sg/taxes/individual-income-tax/employees/deductions-for-individuals/personal-reliefs-and-tax-rebates/cpf-cash-top-up-relief

Spouse/Siblings must not have an annual income** of more than $4,000 in the year preceding the year of top-up (i.e. 2020).

** Annual income includes:

Taxable income (e.g. trade, employment and rental)
Tax-exempt income (e.g. bank interest, dividends and pension)
Foreign-sourced income regardless of whether it has been remitted to Singapore