I thought that his mom is dead (violently killed by the father or something like that) and that is why Sanji is such a gentleman and so passionate about making every woman's wish come true. Sanji can't stand when someone is rude, violent or disrespectful towards women and maybe this is a result of something experienced in the past . I thought that this theory will explain his behavior.
I was re-reading the manga a bit, sanji was actually brought up to never hurt a woman. here is the chapter: http://www.mangago.zone/read-manga/one_piece/mr/one-piece/403/14/
I guess it was his real family who brought him up that way and not jeff because germa kingdom consist mostly of men so woman would be sacred in that kingdom, but that would mean sanji didn't completely abandon all of the germa kingdom teachings.
Or she could have died giving birth to the four kids
I'm wondering who sanjis mom is any thoughts? I have no clue