After becoming the young family head, there were many ceremonies, training procedures and teaching that needs to take place before the young family head gains the power and authority of the family head.
This was to make sure that even if one was worthy in terms of power, they were also mature and knowledgable enough to make decisions for the whole family and taught the young family head most of what they would be expected to know as a family head.
It could take as little as a few days, or could even take years depending on the young family head, but until all the elders accepted the young family head and believed that they were ready after completing all the procedures, they couldn't become the actual family head.
Unless anything unexpected happened, Angela and Cole were guaranteed to become the family head and they were already on track to quickly becoming the full-fledged family heads after their rapid maturation due to Adam.