I haven’t read this but are they all like related? What is this about?

Yes it is about one family, most are related by blood but a couple of characters were adopted/treated like family from an early age or not aware of their true parentage.
There is three main couples.
All M/M.
There is a lot of sexual abuse.
The main story line is about Cinead and Albin.
It's historical storyline with political intrigue and a lot of sex scenes.