Late wife's children from previous marriage

I am in a committed relationship with a Frenchman who has been widowed by a Japanese woman. Both he and she had been married before and each had children from their previous marriages. The children are all adults and none of them lived with the couple, before she died shortly after they were legally married.

His biological children from his first marriage will, obviously, always be his children.

My question is, whether her children (from her first marriage) are legally "related" to the man I am now considering spending the rest of my life with. Are they still his family?

Various rights and obligations may exist if was the acting father/step father during any period...what are you concerned about?

The couple lived and married in Spain, while her Japanese children from her first marriage lived in Japan. They were in their 30s and independent from their mother and her new, French, husband. Today, after the Japanese spouse's death, he still refers to them as his "step-daughters" but he neither lived with them nor legally adopted them.

I am concerned about:
A) Will they always be - legally - a part of his life and thus mine?
B) Will they have any legal claim to his estate in future?
C) Which country's law applies as regards her (and not his) children's legal status? This concerns me because, even though they were not adopted by him the Japanese law regards them as "related by marriage" even after the Japanese spouse's death. Whatever this might entail in practice I do not know and would like to know as well.

I hope it explains and clarifies the situation...and would appreciate your comments.

[able1]

You should have an explicit will made, and have all parties agree because in some cases under Japanese law they will have the ability to make claims on the estate.