The best answer I’ve found regarding Jesus’ family as bookends to this pericope is in Hooker’s commentary in which she relates that the family coming to get Jesus because of his madness is in effect suggesting that they too thought that he was demon-possessed, which of course would have placed them “outside” (v31), with the scribes. I also tend to think that the inclusion of the family is meant to help give Jesus a certain amount of humanity. His family doesn’t see him as whom Mark wants the audience to see Jesus as, but nevertheless, they are still his family with real concerns about Jesus’ spiritual health. This may allow an out for those in the audience who doubt Jesus as well, and for the disciples who continue to stumble for the rest of Mark.
To slander the Spirit seems to be positioned as if one is doing the work of Satan, the Great Slanderer of God’s people. Further, there may be a connection here with Daniel 3.29, in which the Babylonian King, after seeing God’s people delivered by an angel declared to be like the son of the gods. Or, perhaps, as in Zechariah 3 where Satan accuses (slanders) the High Priest Jesus (Joshua; cf. Zech 3.8 with nazer in Isaiah and the use of Nazarene in the Gospels). I’m not sure that the restriction is about denying the work of the Spirit or Jesus’ authority, but rather about standing “in the synagogue of Satan” (cf. Rev. 2.9, 13 and 3.9) and acting the part in opposing Jesus. The difference is that one can recognize that Jesus’ authority is from God, but to oppose him or to speak evil of him is to be Satan.
There are often times a resistance to new moves of God develops in the Church or local congregation. Perhaps the Church is doing something differently, or moving in a different direction than what has previously been done. We put this in terms of liberal or conservative, not realizing that these terms may in fact be slandering the Spirit of God as we are moved by the Spirit of Truth into All Truth (All Truth is God’s Truth). We can either be Christ and destroy the strangle hold Satan has over us and our world or we can be Satan and because it’s never been done this way before, oppose it.