Okay, prophecy deciphering time:
1)Two children, born as one.
Subject: RE:Twins
2)Two Heirs will be born to them.
They will have a successor each. Note TO, not OF, they don't have to biologically produce the heirs.
Note if you kill them a gribbly arising from the corpse or their parents carrying a grudge can count.
If you forbid them from ever marrying they'd just have said heirs out of wedlock.
If you neuter them(pls nobody even think that) they can still go on to raise an heir via mentorship.
As long as SOMETHING inherits their spoopyness its an heir.
3)Blood of own blood will herald betrayal
This is the most flexible part. A few ways to read it:
-Spilling blood of kin will herald betrayal.
--Kill the twins and surprise, their grandfather is not going to be happy at all. Who could have seen that coming?!
--You can also take it once removed, if locals spills greek blood or vice versa, they will be betrayed.
---This is a likely consequence of sending Semni home. A marriage is an alliance. Sending her home means you refuse to acknowledge it.
-Their children(they are blood to both peoples, so the generation after is Blood of own blood) will herald betrayal of some sort. Who's the one betrayed?
--If we do nothing or push for them to keep trying to make the marriage work, the children probably will come to resent one of their parents, which would probably lead to an Accident that is not.
--If we seize the children they're probably going to betray either the man who raised them, or their blood parents, depending on the upbringing.
4) But a union pure will sow greatness
-With the former ominous blood context, the union pure can simply refer to a marriage that WORKS. No bad blood.
--If we make the marriage work somehow, this will be super awesome.
-Pure may refer to racial purity...but the kids are of mixed greek and local blood.
-Pure may also refer to a love-match marriage. Rare, but hardly unheard of.
-Alternatively you can go Lannicest, which isn't exactly unknown to the Greeks(considering Hera and Zeus were siblings)
--But this is squick and hell no to forcing it. We'd probably pretend we didn't see anything if they did wind up that way though.