Dragon Age 2 is so fucked up.

[snarp]

Isabela and Fenris are dating because they bonded over their shared experience of being the only fucking characters who never have to put a doomed loved one out of their misery in a horrific depressing cut-scene.

Aveline - husband
Anders - first love
Merrill - mother-figure
Hawke - mother, potentially also sibling
Varric - potentially brother

Fenris does have the option of murdering his long-lost sister, though! And Isabela can be in the party while you betray her BFF, though he always survives the encounter.