Part of me says to tell mom this is just the way the WTS operates these days. Yes it is extreme and unnecessary but a reality none the less.
Another part says....Sister has laid out some grounds for association so maybe mom can get her to come down on her terms a little by allowing their visit to include a reading from the WT or KM or Bible....
Sister is the one here who is trapped by the WTS mindset and mom would benefit deeply to discern this fact.....Understanding the whys and hows sometimes eases the harshness.
If I was your mom I probably would constantly invite Sister over and keep including her so that she knows she is missed, wanted and loved.
Keep in mind we are still in the midst of DC season....so such uber dedication is going to be abundant