A visitor whose child (above 21 years old) is a Singapore Citizen (SC) or Singapore Permanent Resident (SPR) may apply for a Visit Pass (for long term stay)
网站是这么写的,child-in-law应该不行
P1, P2 Employment Pass holders may apply for Long Term Visit Passes for their:
Common-law Spouse
Unmarried daughters above 21 years of age
Handicapped children above 21 years of age
Stepchildren under 21 years of age
Parents and parents-in-law