@asongofstarkandtargaryen
Grandpa Bernat Estanyol adopts Joan, although he's a single dad raising both little Arnau and Joan, although he couldn't see them grow.
Arnau Estanyol adopts Mar, being at first a single dad and raising her with Sahat who is like Mar's second dad, although he married Elionor, but I wouldn't consider she was a mom for Mar.
Hugo Llor adopts Mercè, being at first a single dad and raising her with Barcha, who was after all like Mercè's mom, but later he married Regina, and she was a good mom for Mercè at the beginning.
The Estanyol/Llor family and their close friends when they see a homeless or orphan child:
It make sense for the characters to do so, all of them have been through very hard conditions like poverty, classism and even slavery in Sahat and Barcha's case. So it's cute that when they have the chance to help other people that have been through hard conditions too or are very vulnerable like poor and homeless children or people who suffer religious discrimination like jews and muslims
Granpa Bernat and little Arnau were treated like shit in general by the Puigs and other nobles because Granpa Bernat was a peasant, but he tried to make Joan's life a bit better than it was.
And little Mar was an orphan of the Fortuny family, a family of bastaixos, and probably she reminded Arnau about himself when he was just a kid.
A great part of the plot around Arnau, Mar & co. is achiving a power position but always remember where you come from and trying to use this position to help people like the smallfolk and the jewish population from other members of the nobility, the whole " We're not like them" thing as it's shown in La Catedral del mar and a bit in Los Herederos de la tierra when Arnau and Mar are trying to help Hugo and his family ( other issue is that although Arnau tried to help them, the positions in which he promoted Arsenda and Hugo's mother didn't turn very well for them) and they truly cared about him because Bernat Jr. and him were very close friends like they were brothers, so Hugo learned their ideals since he was a child.
Something similar is shown when Barcha, Caterina, Omar and other freed slaves formed a community to help each other and trying to help other slaves to quit slavery like Elena the greek girl and other from enslaved people in Barcelona