Half brother and sister related to other family?

Question

After taking an AncestryDNA test, I matched a half brother, who I'll refer to as Y to preserve privacy, and sister, who I'll refer to as X. Both appear to come from the maternal side (both adopted and all family information verifies a male and female child).

My results now show that my full sister, who I'll refer to as S (and I grew up with) is actually my half sibling. I share no paternal DNA with her, but share maternal DNA. We all have the same mother.

However, my half-brother Y and half-sister X do not show my (assumed) full-sister S as a DNA match. A transplant surgeon verified that I am half sister to each of them.

What in the world is going on here?

Why don't Y and X appear as a DNA match to S?

We are confident we all share the same mother. Mother has passed and no idea who the father could be.

Answer 1

Are you saying that X and Y don't match S genetically at all? Or are you saying that X and Y aren't a good match for S in terms of organ transplant? Because those are two totally separate things; any two siblings, even full siblings, can have very different HLA types for organ and/or stem cell transplants.