How and when to use "eius" and "suus -a -um"?

Salvete homines.

I'm doing some exercises through the workbook of "Lingua Latina Per Se Illustrata", and I'm in the Capitulum duodecimum, Exercitium 3.

So, this lesson talks about family relatives, like brother, sister, uncle, etc. And there is this sentence that says:

Aemilia tristis est quod frater eius procul ab ea abest.

So, it's talking about Aemilius the avunculus, Aemilia's brother. But this drives me crazy. Why cannot use suus instead of eius? Isn't she the main subject of the sentence, and isn't he her brother? Because it's not talking about someone else's brother.

Is it more simple than I think, or is it really that difficult?

Valete.