Mary was her mather. Why do we use ”was” not ”is”?

What you are talking about is the biography of Albert Einstein. When you are writing on a person's biography you may use different tenses depending on whether the person / the action you talk about is still alive / exist at the moment you write the biography.
For example Albert Einstein has died already in 1879, so the writer who wrote his biography in the Wiki must use past tense when he talks about Einstein.
As for Einstein's parent, even though they were Einstein's parents forever, but this fact is not present anymore. That is also the reason why we use "was" instead of "is".

Please may I explain more from Baggio(002).

Baggio's answer is very correct. Albert and his parents are no longer here. They died. Read carefully again and you will find Albert was born in 1879. It is impossible a man still living here for 200 years?! (2 centries)

So the conclusion is:

Albert died long ago and we use past tense to write his biography.

1.Like Writing Stories, Writers never write in present tense except for dialogs.

2. Because although this is a fact, but unfortunately was in the past.

3.His mother has passed away