Mothers of Royalty
That means the new princess will remain fourth in line to the throne regardless of whether or not her parents go on to have another son. Versions of the Act were adopted by all 16 Commonwealth realms over which the Royal Family reigns.
Succession to the British throne is determined by descent, gender (for people born before October 2011), legitimacy, and religion. Under common law, the Crown is inherited by a sovereign's children or by a childless sovereign's nearest collateral line.