Ancestry DNA says my youngest sister may be a half sibling

[ancestrydnashock]

I got my updated Ancestry DNA results and only have 26% dna in common with my youngest sister. We are of multiple ethnicities; my dad is 100% X while my mum is multi ethnic. Mum passed away two years ago and dad died decades ago. I look like my father, while she takes after our mum, so no cause to think anything odd.

I'm didn't think much about the results at first, but then did a google search which said, while very unusual, it was possible for a full sibling to show up as a half sibling. I then asked AI and input more info, including the name of the test, then got the answer that the shared cM amount, indicated a half sibling or niece.

The only thing I had found mildly questionable before, was that my sister had 40% and not 50% X dna in her test results and that her dna was from a different part of the country, (think east, not south like mine).

To say I'm shocked is putting it mildly and I have to think about what, if anything, I do next.

Tears was a typo for tests. I do respect that, but these tests aren't just about individuals, they can ruin many people's lives, I there needs to be more understanding of that.

By who? Who do you think doesn’t understand the impacts of finding relatives?

@ancestrydnashock get yourself on DNA Detectives on Facebook and ask for some help from what is called a 'search angel'. Don't mess around on Reddit. It's free and the people are very caring, I unpicked my adoption/my birth father's adoption story that way.

Quite a lot of people think it'll be a bit of fun and maybe they'll uncover some distant cousins descended from great-aunts and -uncles who emigrated to the US or Australia. Then they find out that Dad wasn't the provider of the sperm that conceived them or Mum was actually grandma covering for "big sister's" teen pregnancy and the family gets blown apart.

While that will certainly be unsettling at first, I would say that it is much better to learn the truth rather than to live with a lie (that everybody else in the family knows is a lie except for you).

Frequently, most of the family don't know. A married rape victim may not even be sure herself whether her husband or her rapist is the father of her child. The husband may not know about the rape.

Update.

I've spoken to my youngest sibling and we discussed the reasons why our relationship is showing as 'half sibling'. She took it very well (has a laid back personality and wasn't thrown in the slightest) and said that she had suspected something like this from my message, so we've agreed to keep this to ourselves for now.

We've also agreed that I ask another sibling to do an Ancestry test to help determine which of us has the different father, as a pp suggested. I've only said that I'm doing a family history/tree and looking to connect with paternal cousins etc overseas.

Speaking to other siblings, it's now become obvious that we either have gaps, been told different things by our mum, or misremembering what we've been told.

I also bought the PRO tools, as recommended by a pp.