If an older woman who was never married before, gets married, does she need to separate after the first night if no blood was visible?
She does have to separate. The reason for this is because chazal said “lo plug”- that all women, even if she is a young child that never did yet see, that she still has to separate. An older woman who has already seen is also included in this.
Yad Halevi Y:D 80, Ashrei Hoish 26-12, Pardes Simcha 193-4.