Our stepfather never formally adopted us but our mother had custody of us and we grew up with him. One of his relatives in france has died intestate and we are now possible heirs of this relative. However, we need to establish a legal relationship to our stepfather (deceased) and would like to know how to do this.

What state or country are you in?

To my knowledge, you cannot be adopted posthumously (after the death of a would-be adoptive father) anywhere.

And if French laws are like the laws of most places, unless you are the legal child of the decedent, or if he provided for you in his will, you are out of luck.

we are now possible heirs of this relative

No, you are not. I agree with AK Lawyer

Were you named in your stepfather's will? Is your mom still alive, if not were you named in her will?

Who told you that you 'need to establish a legal relationship?'

Although I agree with the others, I do know someone who is the illegitimate child of a famous and wealthy person (Italian) and who never even lived with him, yet with the help of a lawyer and agreeable relatives was given a sizable amount of his estate after he died. That's more direct, of course, and she was also the only child he fathered.