I think I’ve discovered my mother has a half sister

We are Australian. My mother’s father was a Frenchman who worked as a chef on ships travelling the world and it seems he had a girl in every port.

We knew he had a wife and child in France when he met my grandmother and had my mother.

Ancestry DNA is showing a woman as my first cousin with 691cm. Our only shared connection is my mother. No other relatives in common.

Both this woman and my mother were born in the same year. The woman in March, my mother in December.

Does this sound the most likely? That they are half sisters or am I missing something?

Edited to add: the half sister who I will refer to as AG has her tree public. It lists her father as a man named NG, no mother and NG’s parents. However I’ve found the mother/wife who I will call EG. I’ve found the birth notice of AG with NG and EG as the parents.

Interestingly NG, EG and my grandMOTHER were all in the armed services together at the same time.

So I think the other possibility is that my mother’s father is not the Frenchman but NG.