I can't say for sure, but the hair color is the same, in the RTF we saw only 3 humans with white hair. It could be that Cyrus' mom got implants or body parts replaced, so far there is no confirmation. Let's wait for chapter 14, maybe this chapter will at least explain something to us.
I really don't think it's his mother. Cyrus' hair originally isn't white. It turned white while he was kept in "hibernation" or whatever. This just indicates, along with the "ancient" stuff in the room , that she is most likely from the same time period as Cyrus (give or take a year), and they are in similar shoes.(The girl is also young, i don't think "hibernation" can reverse your age, but it sure can "freeze" it) I think it's either Diana under a different name for whatever reason (due to the hairstyle and the plants), or a new character entirely (perhaps related to her?).
I think there might have been a confirmation. Looking back to the letters damion, diana and his parents wrote to cyrus, only Diana wrote something interesting. "I hope to see you soon". It's just a theory, but i think this is also one of the clues indicating that this person is diana. add to that the taking care of the plants on her room