--SPOILERS---
I'll transcribe it and people can decide for themselves, but I thought it was pretty clear.
Midgardsormr came to the planet (space dragon) with seven eggs, when they grew up they went off to different regions to populate them.
Here's the transcription
Tiamat: Harken to my tale, child of man. I am Tiamat, pf Midgardsormr's first brood.
Tiamat: In a time before time, my father did come to this star, bearing seven eggs.
Midgardsormr: From these eggs did my children hatch. And once they were full grown, they took wing and spread across all the lands of Hydaelyn.
Tiamat: Together with my brood-brother Bahamut, I journeyed south to Meracydia. And together, we brought forth innumerable children unto the world.
Tiamat: We abided in peace, and all was well...until the men of Allag came, some five thousand years since. They slaughtered my children in droves and took from me my beloved Bahamut.
The dragons of Meracydia were all her children (and she brought those children together with Bahamut). All dragons come from Midgardsomr.
I don't think this is read any other way.
On another note, I think this might imply that Ratatoskr wasn't just Nidhogg's sister but his mate as well.
His rage would be justified either way, but this adds another layer to it.
I wouldn't be surprised if the 6 of the first brood all paired off while Hraesvelgr was the odd one out. But he found companionship with Shiva.
Well that last part is speculation, but the Ratatoskr being Nidhogg's mate is very likely in my opinion.