I'm replacing sill plates and rim joists. All of that is going well.

I have several floor joists that have semi-rotted ends. The ends are rotted about 1&1/2 inches into the end of the joist. After that it's good wood. I've done one sistering on those. I sistered the existing joist with a 12 foot section of 2x10, taking it to the center support thingy. Is that overkill? Can I just use smaller lengths of sister 2x10's? Do I have to do the whole 12 feet for the 8 to 10 joists that have that small area of rot?

Any advice is greatly appreciated.

To be properly "sistered", joists must bear on the same surfaces as the original joists. That means that to do the job properly the new joist must rest on the plate and the beam.

The sistering of the floor joist to the center support beam in the basement is not overkill at all. Its the ideal way of doing this type of project.
However depending on the general condition of the floor joists your sistering, the amount of knots in the wood the type of wood used the amount of holes drilled into it will determine how far you need to go on the sister joist. Ideally you should always go beyond the damage by at least four feet. There are specific nailing patterns or bolting patterns that are required when doing this in this fashion in order to allow the new sister joist to properly connect to the old. If you go from sill to top of center beam the amount of support fasteners drop as this new joist now takes up the majority of the load. The fasteners simply need to be placed so the beam does not roll over or squeak due to movement.
Always use a good quality contractor’s adhesive when sistering joists as well. This assures a good connection between the two joists.

Fantastic advice........Thank you both very much. I now have what I needed to know.