So, if the feds came for one of my family members, I'd be asking for it, and paying the price, if I got shot in the head? It was my choice be with them, standing as I might in the house 'n all. Sure. Scary, but not really sarcasm, is it. In this media world, it's possible that nobody would shed a tear for me. But I shed a tear for Vicki Weaver. Remembering is the best thing, which is the whole point.
They've been a little stickier about such claims advanced in peacetime, or at least they were when John Marshall wrote the opinion in Ex Parte Bollman and Swartout, a habeas corpus case involving two men accused of (and imprisoned for) treason in the Aaron Burr adventure.
When Roger Taney wrote an opinion in Ex Parte Merryman during the Civil War, involving a Maryland Militia officer who was carrying out orders of the governor of Maryland that displeased President Lincoln and his generals, Lincoln ignored Taney's opinion freeing Merryman from imprisonment, continued to hold Merryman, and wrote out a warrant for the arrest of Chief Justice Taney which he gave to confidential agent Ward Hill Lamon, who never served Taney but instead, in the absence of any followup by Lincoln, held the warrant for a time and finally disposed of it. Lamon mentioned it in his memoirs, however, and Taney did know about the warrant, which he mentioned to someone else.
In your hypothetical, the "fortress doctrine" would only come into play if your kinsman opened fire on the lawmen and forted up in the house, like the Branch Davidians did. Weaver's case was a little iffier, since he never opened fire on lawmen -- but he did bid them defiance.
If your kinsman resists arrest and takes refuge in the house, the law (I think) requires you to bail from the premises immediately to avoid any animadversion of the law on yourself. The law requires you to abandon, rat out, inform on, and otherwise turn on your blood kin instantaneously, the second it moves against him.