Yellow Ice Meadow
These things are all the same...
Your first mistake was having a character with a backstory. I only give my RPG characters backstories if I think the stories are compelling enough to be worth the hassle of the GM inevitably kidnapping or killing everyone in them. (Other than hated enemies, who inevitably show up and single you out)
Well, too late to fix that. On the bright side, after your character loses everything, the GM will run out of things to fuck up. Just try to avoid letting the BBEG take anyone prisoner. Then they'll never ever stop torturing them and claiming it's your fault.
>As long as you've not crossed that line yourself, the BBEG will be out of line in attacking family to get at you and you will have cause to ask Wizbro for help
The BBEG apparently tortures and slaughters people for fun. I don't see how torturing and slaughtering people for flimsy, petty reasons is somehow worse than doing it for no reason at all. Moreover, unhelpful asshole ultra-NPCs never step in unless the GM feels threatened, or it's part of some big boring NPC vs NPC plot point they planned way ahead of time.
Anyhoo... Best advice I have is to try and figure out why the BBEG didn't bother crushing this place beforehand, and amplify that. Maybe hit their base of operations while they're in the middle of the siege, or spread fake stories about how you're headed to find an ultimate doom artifact, or whatever.
Also, be willing to kill off your character's loved ones to prevent capture. It'll make you look like a dedicated, motivated hardcase, even though it's actually just convenience.