Why can't my adopted sister accept she is not part of my family because she isn't related? Why can't she stop calling my parents mum and dad?

When my son found out that his Aunt M was technically his step Aunt and not not blood related, did it change how he felt about her? Not. At. All.

My son is 13. Earlier this year, we'd gone to visit my family as we usually do several times a year. My two sisters, myself and my son were sitting out on the back patio, just talking and catching up. I'd noticed that my son was being unusually quiet and had been looking between my two sisters and me. When one of my sisters went inside to talk to our dad, my son said “Mom…why doesn't Aunt M look like you and Aunt K? I mean, you and her look kind of alike, but not as much as you and Aunt K do.” I told him it was because his Aunt M is our stepsister, which was met with utter astonishment and a bit of indignation on his part. “What?! Why didn't you ever tell me that??!!” I explained that I probably never mentioned it because, to us, she's just our sister - we don't think of her as our stepsister. The only time that we describe her as our stepsister is if the relationship needs to be clarified as such. Other than that, M is our sister. Plain and simple.

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What does the Bible say about the Antichrist? How will we know when he arrives on the scene?

If you weren't aware that your sister was adopted, would you feel the way you do? My guess is that no, you wouldn't. It's time you did a little soul searching to understand why it bothers you so much and why you're so focused on the fact that she's adopted instead of realizing the gift you have in having a sister.

Oh - the answer to the last part of your question (why she “can't stop calling them Mum and Dad”)? It's because they ARE her Mum and Dad, too.