Game news:Royal Relatives
You can be a sibling to the current kings and queens but those siblings will not likely be able to 'take the kingdoms' from the direct lines (Jaden, Missiah, Kalem, and Elena). These siblings, be it grudgingly or willingly, support their kings and queens. All royals are strongly aware of their bloodlines and strive to uphold the 'traditions' of their kingdom, even if they absolutely hate it or want to be the 'direct line'. The children of the siblings can even resent their cousins for being able to become 'king or queen' in the future. Even the children of the direct lines can resent the sibling that is the 'heir' while doing the 'proper thing' and showing support for them. There is the possibility of the direct line siblings plotting to over throw the 'heir' of the kingdom but we will expect it to be more against the NPCs over the PCs, unless the target-PC explicitly lets staff know, via request so there is a record, that they accept being over thrown by their sibling.
For the royal families we tried to put everyone at ages that would allow for plenty of room to be part of them quite easily. We are willing to talk about someone that has been adopted into the royal families but are not blood related to Kalem, Elena, Missiah, or Jaden. No, it can't be someone that started as a Commoner, with the exception of the commoner being a Child of Gods. They could be a sibling of any of the spouses (it is easier if it is a spouse that does not come from another family currently created).
Keep in mind, that we do have restrictions on how many PCs are in each family in the interest of spreading out players across all families.