FULLMETAL ALCHEMIST BROTHERHOOD Facts

Ed is a prince! Sort of....

So this will be one long fact.

Hohenheim's blood was used to create 'father'. This makes them practically brothers, they both had kids.
( Humonculi and the Elric brothers ).

If father and Hohenheim are brothers then their children would be cousins.

When Greed 'Father's' child becomes one with Ling they are the same entity.

So Greed is Edward's cousin this makes Ed a person of royalty in Xing.

BUT!

Father and Hohenheim are indeed different beings. But they were not born from a mother. Father was created from the essence of Hohenheim. This makes them the same person. So if this was then their Children would merge.
Making Greed and Edward brothers. Then Ed would be a Prince.

But technically Hohenheim and Father are different beings.

So Ed and Greed would just be cousins........