Howdy,
I had a similar crack in my walk ins, was losing quite a bit of water. Here's what I did: (my steps are plastic, not fiberglass)
Drain pool below level of crack, mine was the second step, so no liner issues with the level down.
Drill out the ends of the cracks with a small drill bit. This will stop the crack from spreading.
I used a dremel tool to widen the crack enough to inject Great Stuff to fill the void between the steps and dirt.
I pumped a couple tubes of the epoxy plastic filler material into the crack once the great stuff had dried.
(These two steps could be improved upon I'm sure. There may be a better way to shore up the step)
Fill the crack in with flexible epoxy, or caulk. If it doesn't flex, it will crack again under foot traffic.
I then cleaned everything up (lightly sanded) and painted with white appliance paint.
Let everything dry completely before re-filling the pool.
If you were very enterprising, you could build a dam out of stout particle board and pump the steps dry without draining the pool. Not sure how well that would work.
Good luck with whatever you decide to do!!