Hi Paul,
If it's been in the same mix for several years, than it's probably due to be re potted. Especially if it's in a mix that has a large amount of peat.
Over time, the mix is going to collapse, and not allow for proper air and gas exchanges, which are vital to the tree's vigor and healthy roots.
Alot of the members here re pot their trees 1-2 years.
Below I've provided a link loaded with information that will be a good starting point to understand how tree's in containers are grown.

I hope it helps! It's a good start until others can get here to help further. :-)

JoJo

Here is a link that might be useful: Tree's in Containers

Paul your sister is in the majority with those growing citrus in containers. Citrus is pretty tolerant of collapsed soil, as long as she is diligent in her water/fertilizer routine. Of course her mix should be changed. I have not seen a timed release fertilizer among those formulated for citrus. If she is using the miracle grow labeled for Azaleas it works OK as long as she does it on a monthly routine, year around. Al

My sister had been growing her grapefruit which is as tall as her ceiling in the same pot without a transplant for more than 10 years!

She use to just scrape out the top few inches of the mix and then replace it with fresh stuff.

This year it croaked as fast as lightning can hit you off guard.

I change mine out every couple of years or so, unless the plant outgrows their pots.

Mike

Mike