
Sooo Miriam is his own mother? Or was she just a concubine to the king and not his mother?

Miriam is his first wife. Since he was so sickly as a child, his mother made him marry Miriam to make an heir so that even if he dies, they can still be in power as opposed to him dying young and childless thus allowing other relatives of his to have a claim to the throne, thus leaving his mother and his wife's family at risk of losing power.
That's how I understand it from my reading.
Hello there Sir Eren ლ(´ڡ`ლ)