I think once a twin, always a twin. A twin is a person who was born at the same time to the same mother, and that doesn’t change when one dies. Obviously, a twin who’s twin died as an infant may not “feel” like a twin, and may never or seldom refer to his twin, but that’s a personal choice.
The niece question is harder for me. Even though I’m married, I often refer to my husband’s brother’s kid as “my husband’s niece”, because I don’t feel much of a connection with her. If we were to divorce, I’d probably refer to her as “my ex’s niece.”
And aunts’ spouses are always uncles; not aunt’s husband. And I call my husband’s niece “our” niece. I just grew up in a family that was all-inclusive on that stuff, so that’s how I do it. My cousin’s wife is my “cousin in law.” Goofy, I know…
Everyone is always either the son or daughter of someone; we use these terms to describe lineage regardless of deaths. But is the word “twin” dependent upon the existence of another person. If, during the birth of twins, one of them should unfortunately die, would you call the surviving child a “twin”?
I ask this question only to explore the semantic aspect of the term. Is it about biology, or is it about identity?
If my twin died, I’d still think of myself as a twin. It becomes a part of your identity because the relationship that twins have is usually different than other sibling relationships. That experience doesn’t go away just because your twin died.
My mother had a twin brother who died after less than 3 days, and my fiancee’s father has a twin who lived. Both of these people refer to their twins and consider themselves “twin” to someone. In my experience, whether or not the twin is living one may still consider themselves a twin.
As a side note, clearly my fiancee and I are expecting that I will carry twins when we choose to start a family. It would fit our families perfectly.