Following the lead of the Scandinavian and Benelux countries, the UK decided to change the laws of succession to let the Crown pass to the senior heir of line, whether male or female (and repeal the laws disqualifying Roman Catholics and those married to them from inheriting the throne). But abolishing the rule of male primogeniture isn't so easy when you're dealing with someone who is not only Queen of the United Kingdom but also of 15 other realms (in constitutional parlance): Canada, Australia, New Zealand, Jamaica, Antigua and Barbuda, Belize, Papua New Guinea, St Christopher and Nevis, St Vincent and the Grenadines, Tuvalu, Barbados, Grenada, Solomon Islands, St Lucia and The Bahamas. Commonwealth prime ministers agreed in October 2011 that they would make the necessary changes in their respective countries, retroactive to 2011, which is easy to do in unitary states (simple enactment or even Cabinet order in some of the smaller ones), but more difficult in federations like Australia or Canada, where state or provincial consent may be required.
Until all 15 non-UK realms get things sorted, there was some risk that Canada could end up with a different sovereign than the UK – or the Solomon Islands. The fact that the new baby is a boy means, however, that for the time being there will be no Royal sibling rivalry.