NotFormer : the 144,000 are (or were until recently) the FDS.
As discussed previously, the 144,000 are in heaven. Those who are anointed on earth are referred to as the remant, the remaining ones. These remaining ones were (until 2012) understood to collectively be the FDS.
NotFormer : [The remnant] are Jesus ' brothers?
Correct. Matthew 25:31-46 makes a distinction between Jesus' brothers and sheep-like ones who support them. See, for example, the article Loyally Supporting Christ's Brothers.
NotFormer : [Every JW] who is not the [remnant] are the "other sheep".
Correct.
NotFormer: [The "other sheep"] are not Jesus’ brothers.
Correct, in the sense of Matthew 25:31-46.
NotFormer : Yet the GB, who are Jesus' brothers, refer to [the "other sheep"] as brothers and sisters. But how can that be?
Technically the "other sheep" are not brothers and sisters of Christ in the sense of Matthew 25, but they are still addressed as brothers and sisters as they share the same faith. I suppose the language is borrowed from Paul's letters without being pedantic that when Paul wrote he was writing to the brothers of Christ.
NotFormer: The [remnant] are from Jesus' family, but the [other sheep] are not part of Jesus' family.
Scripture only makes a distinction between Christ’s brothers and sheep-like ones. It doesn’t go beyond that.
NotFormer : If the great crowd is not related to Jesus, then who is their father?
The "other sheep" are not Jesus' brothers but God is the father of all.
NotFormer : If the GB are "our future kings", then shouldn't the other sheep...be referred to by the GB as "our future slaves"?
We are all slaves of Christ, are we not.
NotFormer : What now is the status of the [remnant]; do they matter? Are they the GB's brothers and sisters?
All members of the Governing Body claim to be spirit-anointed and are thus part of the remnant. So, yes, according to scripture support given to Christ's brothers on earth (i.e. the remnant) does matter. But I see your point that Matthew 25:31-46 was more pertinent when the entire remnant was understood to be the FDS.