Other words to describe the relationships between pieces of data with a common parent/child

The term "sibling" is common usage in this context, yes.

I haven't heard "co-parent", but "parent" is the usual term in a tree structure, so "co-parent" sounds about right.

It’s hard to tell how to answer this well without a better understanding of the relationship you’re talking about.

1. If “A → C” means “A is like C except for (some factor)”, it seems as though the relationship is not just not hierarchical; it is anti-hierarchical, and symmetric, so it should be written “A ↔ C” and “B ↔ C”.
2. If the relationship between A and C is “A is like C except for (some factor)”, does that mean that A is like B except for that factor? If so, the relationship seems to be transitive, so “A ↔ C” and “B ↔ C” implies “A ↔ B”, i.e., the relationship between A and B is the same as that between A and C, and between B and C.
3. Or does it mean that A is like B, even considering the factor (e.g., a lion is like a cat except for size; a tiger is like a cat except for size; a lion is like a tiger.)?
4. Or does it mean that A is like B, except for two factors (e.g., a lion is like a cat except for size; a lion is like a bear except for genus)?

Based on what you’ve given us, if case #3 applies, these might be what you want:

• A and B are comparable
• A and B are in the same cohort
• A and B are cohorts         (although some might say that this is an incorrect usage of that word)

And if case #4 applies, consider

• A and B are cousins (or distant cousins)

In any case, when talking or writing about this, you would need to define the term you are using.