Subjects of Ymir are all descendants of Ymir. What differs the so called royal bloodline from the rest?
That's actually a really good question. I suppose you are talking about how only the royal bloodline can use the full potential of the Founding Titan. My theories as a somewhat mediocre fan to solve this apparent plot hole are as follows:
It actually doesn't matter from what we have seen in the manga. Eren was able to seize control of the Founding Titan by convincing Ymir, the First Titan, to make her own choice as she was a slave to the will of the royal bloodline since the days of the founding king, Fritz. Hence the royal bloodline wasn't really special. They just had the advantage of manipulating a nigh-omnipotent girl to do their bidding.
The first theory can be shot down when you consider the fact that Zeke's spinal fluid can be used to turn Eldians into pure Titans. Thus perhaps the royal bloodline is special. I'm no geneticist, just a hyperactive high schooler, so what I'm about to propose may be scientifically preposterous. I think because since the royal family is directly descended from Maria, Rose, and Sina, the first humans to eat Titan blood raw I might add, it's possible that there's a higher concentration of the original power in the royal bloodline hence their abilities. How you might ask? They may have taken a page from the Targaryens.
I hope this answer is ok. If I'm wrong somewhere, please correct me.
Plain and Simple,
What we know is that in the Beginning there were Many tribes and Eldia was the dominant one and it was lead by Master Fritz (A power hungry man). So, one day he hunted one of his Servants Ymir, for letting of a pig, who ran to the forest and receives the Titan powers from a Spine like Creature. Seeing that Ymir would be useful, Fritz made her do all the work of his nation, like creating Bridges and fighting wars and made her his Concubine and had three Children. They were Rose, Maria, and Shina (Whose names are the Three walls), So when Ymir was killed, sacrificing herself for Fritz. Fritz made his three children eat Ymir dead remains, and kept the Power of Ymir's Titans. Eventually Eldia Multiplied and prospered with the Help of their Titans and captured many other countries like Marley, bringing hate on them in the Future.
So in one sentence the Royal Bloodline are the Children of Ymir and Fritz and the normal eldians are the ones that were Kings Fritz' subjects and Servants and other children.
If you think there is something Wrong or if you have other Theories and aswers please correct me.