Yes, Fairy Thing is 6th Spawn while Alice is 6th Spawn Spawn.
Ether = The 6th = Grandfather Thing
Fairy = The 6th's Son = Fairy Thing
Alice = The 6th's Granddaughter = Fairy's Daughter Thing
Not sure how Hex and Nico fit in with the 6th. All we know is it's Fairy Thing's Kin and by extension related to The 6th. It could be Fairy Thing's Brother. We do know Hex is an S-Rank Aberration of his kind and much more powerful than the A-Rank's of his species. Nico also has more advanced teleportation than Hex and likely his species but she can't create dimensions like him. So I'm guessing Hex is a high generation. Likely 1st or even 2nd at worse. Which makes Nico 2nd Gen or 3rd Gen of the 6th. Nico is already stronger than Hex too, beating it in their fight.
Aglaecwif is a wild card. She's definitely powerful, being a S-Rank and enemy to Valravn. Is she our sister or The 4th's granddaughter aka 2nd Gen? If she's the 4th's Daughter Thing, she should really be a huge deal. We know her Memory Trait is potent and possibly extremely powerful if my theories are true. So I can't see her being below the 3rd generation. Especially when even the MC couldn't tell she was using her memory trait on him. But she implied that she could possibly have a mixture with the 7th. If she does, that would also explain why Val hates her since she would be a heretic for not being loyal to their Dark Lord.
Anyway, I got sidetracked from the 6th and his kin. The incest in this game is off the charts when you think about it. Though, it's likely not as literal as I'm making it out to be. But it's fun to play around with the thought.