My partner referred to his stepfather...as his stepfather (now dead for past decade). In his heart, he felt closest/admired his birth father who died when he was 2 yrs. old.

The reality was that he never really cared alotfor his stepfather, who was totally different from birth father and how stepfather treated his mother. (meaning he had to protect his mother from his violent tempers...)

And this despite the fact he was raised by mother and stepfather for the remainder of his childhood/teenagehood.

It's interesting about the proper use words for blood vs. in-law family relationships. When my mother learned that one of my sisters referred to her mother-in-law as "mother", my mother was irritated/annoyed. Most likely she might have felt hurt/jealous or whatever. I think she would have been happier that my sister refer to her mother-in-law by anything else but "mother".

In Chinese language there are specific words to identify the maternal vs. paternal and degree of blood closeness for family members. ie. there is a word for maternal third cousin or the birth order of a person in relationship to yourself. I am Elder sister ..and of course always would be because I am the oldest.

But ie. Sister #3, she would be referred by me as younger sister. But Sister #4 would refer to Sister #3 as Elder sister. But for the past few decades, all that stuff gets dropped and it's just "sister".

However we do use words to identify, maternal grandfather and maternal grandmother, vs. paternal grandfather and paternal grandmother. So at least the grandparents on both sides, each have unique terms.

In the situation of step family members, the proper etiquette is driven by how the person feels in relationship to the step family member. Most likely in a traditional bound Chinese family, it is to use "mother", even though a person may not care much for the 'stepmother' or whatever.

Remember Chinese culture has a long history of blended families... for the wealthy, more than 1 wife ..and then there may be the mistress or concubine. Too many stories of interfamilial conflicts and buried anger/resentment/slights.

I think it is healthier for stepfamily relationships, if you don't feel close to the step family member, is to address the person by first name or a unique family endearment that the person likes. Or if you wish to acknowledge who is genuinely your birth parent.