Ch. 137 'Find A Clue'
Merylin, as the successor to the throne of Mermaladia, would not be able to appoint another mermaid to be her sibling. If there were another mermaid who becomes her foster siblings, then there was a possibility that there would be a struggle for the throne. The crown of Mermaladia could be inherited on any mermaid as long as they have a royal member's blood.
If there were a mermaid who wanted to foster a mermaid child or made them into a family member, an adoption ritual must be completed. The mermaid who wished to adopt must give her blood to the mermaid she wanted to take.
The blood that merges into the adopted mermaid will automatically have the same blood as the adoptive family.
Merylin will also give her blood to her adopted younger sibling, making her sibling have the same right to ascend the throne. That was why Merylin could not take her fellow mermaids to be appointed her younger siblings.