This series is the Perimeter One Adventures by David Ward.
Looking for a late 90s middle grade Christian sci-fi book series...
The books were all published in 1994 by Thomas Nelson, a Christian publisher in the United States. They focus on the members of the Graham family: Nathan and Millie, the parents, and Chris, Ryan, and Amie, their kids. Nathan works for SAFCOM, a solar-system wide communications company.
The first book climax involves a spaceship shooting through hyperspace rings of decreasing diameter
One of the major plot points (while not the climax) of the first book, The Misenberg Accelerator, is when a passenger spacecraft is pulled into the titular accelerator, which sucks them through a series of rings that decrease in diameter.
The second book involves a humanoid robot/android which gets its skin melted off by acid in the end.
The second book, The SHONN Project, is about the creation of a sentient humanoid robot named SHONN by Chris and other students and teachers at his university. While they believe it is just a research project, the university actually created it for a weapons company, where it was supposed to be the perfect assassin. SHONN sees a student beaten by a representative from the weapons company, becomes violent, and is eventually drawn out by flushing the sewer and other subsystems with dilute acetone, which would melt its skin but not destroy it or any of the substructure.
The third (or fourth?) book involves caves on Mars;
The fourth book is titled The Mines of Venus. It is set on Venus, not Mars, in a mining colony in domes on the surface, although a large section of the novel is set in caves under the surface.
... there's a creepy blind woman ...
Behind the altar crouched an old hag of a woman with stringy gray hair and a milky white film in the sockets where her eyes should have been. She leaned through the putrid fog and hissed at the two intruders.
The Mines of Venus, pg. v
... a snake ...
Sadly, I could not find a snake.
... and a cultish group which believes they must cut themselves because "by his stripes we are healed" ... The dad and son meet this group, and they're going to be initiated, but the dad points out that it's "by HIS stripes we are healed", not by inflicting wounds on ourselves.
This is almost exactly what happens. Nathan, Millie, Chris, and Amie are going to be initiated and Nathan stops them from doing so.
Gamaliel the chief priest stood and moved to Nathan's side, taking out his dagger. He removed Nathan's sleeve and placed the point of the dagger against the skin of his shoulder.
"By His stripes we are healed."
Before the others could respond, Nathan interrupted. "Wait!" ... "The priest just said, 'By His stripes are are healed.' It is His stripes, not ours."
The Mines of Venus, pg. 65
I believe the showdown with the blind woman and snake also involves some kind of spiritual conflict.
This is very literally true.
At the name of Jesus, the hag clutched her head, and dropped to her knees. All around the hag's chamber, shelves collapsed and tables were overturned, as if the forces of Light and Darkness were fighting a pitched battle in the room. Millie grabbed her sons' hands.
The Mines of Venus, pg. 147-148
I'd also check the covers for each book to see if they look familiar.
Click images to embiggen.