This list looks quite complete, at first glance only Nocte Obducta's Nektar Teil I and Nektar Teil II come to my mind.

Missing Guns 'n' Roses.

^ I feel literally ashamed now.
There's most probably no better example of sibling albums than the classics Use Your Illusion I + II.

More true siblings (recorded in one session and sharing the same story):
Satan's Host - Pre-Dating God Part 1 + Part 2
Benighted In Sodom - In Hora Maledictus - Part I + Part II
Solefald - An Icelandic Odyssey (Red For Fire + Black For Death)
Senmuth - too many siblings to get hold of them. Senmuth's discography is one single shameful sibling incest.

Not sure about
Stratovarius - Elements Part I + Part II
Both albums are from 2003 but I will not dig any deeper unless someone is willing to pay me. Just can't stand this band. Sorry.

Vanden Plas - Chronicles of the Immortals - Netherworld (Path 1) (2014) and Chronicles of the Immortals Netherworld II (2015)

pretty good power-prog

Missing Rebellion The History Of The Vikings Part all 3 albums

Vintersorg till fälls

You can put every single The Ocean album on here, right from the start , till their latest...

How about Amenra's Mass I - VI?

Written by Paz on 03.07.2019 at 08:39

How about Amenra's Mass I - VI?

Too vague

There's also the Deathspell Omega trilogy.

Written by Mercurial on 03.07.2019 at 13:26

There's also the Deathspell Omega trilogy.

The fact that I don't know for sure which are supposed to be the three DsO albums in the trilogy is reason enough not to include it

STOP!!! You MUST check out the progressive metal band called Tomorrow's Eve. They have three sibling albums called Mirror of Creation. A severely unfairly underrated band that you will be astonished by and must include these albums in your list. Thanks.

What about Edge of Sanity's Crimson and Crimson II?

Written by F3ynman2000 on 01.09.2021 at 22:49

What about Edge of Sanity's Crimson and Crimson II?

"So like albums that have a clear tie between them, were recorded around about the same time and were clearly intended together, as opposed to a sequel coming years after."

Written by RaduP on 02.09.2021 at 00:21

Written by F3ynman2000 on 01.09.2021 at 22:49

What about Edge of Sanity's Crimson and Crimson II?

"So like albums that have a clear tie between them, were recorded around about the same time and were clearly intended together, as opposed to a sequel coming years after."

Ah ok, fair enough

You're missing Old Man Gloom, who released two albums on the same day with the same title and very similar cover artworks. Seven years later I still think that's hilarious.

Therion Sirius B & Lemuria

Iced Earth - Something Wicked Pt. 1 and Pt. 2

Therion : Leviathan I (2021) and Leviathan II (2022)

Blackbraid: Blackbraid I (2022) and Blackbraid II (2023)

Written by Plattner on 27.04.2023 at 16:48

Blackbraid: Blackbraid I (2022) and Blackbraid II (2023)

are they siblings albums though? By this logic, Led Zeppelin I and LZ II should be here.

Written by JoHn Doe on 27.04.2023 at 16:50

Written by Plattner on 27.04.2023 at 16:48

Blackbraid: Blackbraid I (2022) and Blackbraid II (2023)

are they siblings albums though? By this logic, Led Zeppelin I and LZ II should be here.

Your question is valid. Because these are the first two albums by Blackbraid, the numbering of the albums might not necessarily suggest a concept as siblings albums.

However, the mentioned Leviathan albums by Therion are indeed siblings albums.

Written by Plattner on 27.04.2023 at 17:18

Written by JoHn Doe on 27.04.2023 at 16:50

Written by Plattner on 27.04.2023 at 16:48

Blackbraid: Blackbraid I (2022) and Blackbraid II (2023)

are they siblings albums though? By this logic, Led Zeppelin I and LZ II should be here.

Your question is valid. Because these are the first two albums by Blackbraid, the numbering of the albums might not necessarily suggest a concept as siblings albums.

However, the mentioned Leviathan albums by Therion are indeed siblings albums.

The Therion albums are siblings indeed.

Written by Zap on 02.09.2021 at 12:14

You're missing Old Man Gloom, who released two albums on the same day with the same title and very similar cover artworks. Seven years later I still think that's hilarious.

I will as soon as I figure out how to tag each of them.

Lords Of Black:
Alchemy Of Souls - Part 1 (2020)
Alchemy Of Souls - Part 2 (2021)

This could potentially be another pair:

TesseracT:
Concealing Fate (2010) [EP]
One (2011)

The chronological proximity, the artwork and the fact that the title of the EP is a song on the album may indicate that these are siblings albums.

However, I have not checked literature, interviews etc about this to be completely sure.

Royal Hunt Dystopia and Dystopia -- Part II

Saviour Machine:
Legend Part I and Legend Part II
as well as
Legend Part III: I and Legend Part III: II

Neurotech:
Decipher Vol. 1 [EP], 2012
Decipher Vol. 2 [EP], 2012
The Decipher Volumes, 2013 [contains Vol.1, Vol. 2 and novel Vol. 3]

W.A.S.P. : The Neon God Part 1 and Part 2 (both 2004)