I was told by my mother-in-law that my wife should follow all of my minhagim except for shabbat candle lighting and mikveh, for which she will follow her mother's.
Is there any halachic authority who rules like that?
It would seem that a woman does not need to follow her husband's minhagim for things that are considered ladies' mitzvos. Thus R' Moshe Feinstein (Igros Moshe EH 2:12) writes that it is up to a woman if she wants to wear a sheital (wig), and she doesn't need to follow her husband if he feels it's not good enough as this is one of her mitzvos.
Rabbi Doniel Neustadt writes that there is justification for following one's mother
It seems that there is a halachic source for women following their mother's custom. To prove this point, let us examine a well-known custom which is connected to the mitzvah of candle-lighting:
It is customary for many women to recite the blessing of shehecheyanu when they light candles for Yom Tov. Although this custom has no source or basis in Halachah - indeed, it may be halachically objectionable - it has nevertheless become almost universally accepted.
Rav Yaakov Emden reports that he, personally, objects to this custom. Indeed, he rules that if a woman does not have the definite custom of reciting shehecheyanu at candle-lighting time, she should not do so. Nevertheless, he says, his wife - who saw this custom in her parents' home - does so, and he does not object. Since it is not halachically forbidden, he does not feel compelled to reject her minhag which she witnessed in her home.
Surely, Rav Yaakov Emden was aware that upon marriage, a woman ought to change her customs to follow her husband's. Still, he did not insist that his wife abandon her mother's custom and adopt his own. Perhaps Rav Yaakov Emden held that customs pertaining to candle-lighting are an exception to the general rule. Since, as mentioned above, our Sages made it the woman's responsibility to light candles, it becomes "her" mitzvah, to be followed according to her customs. Apparently, it is not incumbent upon the husband to insist that his wife alter the customs which she learned from her mother. Although she may do so if she likes, she is not required to do so.
See this answer https://judaism.stackexchange.com/a/70524/5120 in my understanding of the reason your case should not be different
What the above answer quotes regarding a wig, does not answer the question since it is not regarding the husband custom but only regarding his complaint about his wife's behavior, and that she has on what to rely to not have to listen her husbands complaint regarding her religious observance
I guess that even acording to Rav Moshe Finstain, if an ashkenazi girl marries a sefardi man she will be forbidden to us a wig
And the story with Rav Yaakov Emden is also not regarding the husband's (mother's) customs but only the husband's halochic decision,
I learn from it that a husband maybe should not inforce his innovations (unless it is really forbidden) on his wife, she can keep her mother's custom
(It is possible that Rav Yaakov Emden's mother also did not say the brocho, (that would change my understanding of the story quoted here), but still it might not mean that his mother did not do it because it was forbidden, so the only thing happening here is that his wife is not doing anything bad by doing something extra)
In Halichos Shlomo, in chapter 9 of hilchos Succah, footnote 37 Reb Shlomo Zalman Aurbach is quoted as saying a lady should light candles in accordance with her mothers custom.
Even though most customs should follow her husband's customs, concerning lighting candles which is a mitzvah for women (מצות הנשים), all women should act in accordance with their mothers house.