In Yechave Daas and Chazon Ovadia, Rav Yoseph Shlitta says that when a women makes the bracha of lighting candles yom tov night she should not say shechiyanu by the candles. He says that since one can say shechiyanu at anytime on yom tov and there is no reason to say it by candle lighting time, therefore saying shechiyuna then is a hefsek between the bracha on the candles and the action of lighting the candles. I have noticed that Ashkanazi poskim like the Igros Moshe have used sayings of Ein Limchos (don’t rebuke) women who have the custom but they don’t go as far as saying that the minhag is wrong as Rav Yoseph Shlitta says. My question is why is this not a hefsek? Is just that this is the “minhag” a strong enough reason to not rebuke the custom?
One can suggest that the shehecheyanu blessing would not constitute a hefsek because the blessing is made in honor of the day, and the lighting is also in honor of the day, so that the blessing is suited to the lighting and does not form a hefsek (see She’elas Yaavatz 107; Zera Emes 1:33).
Another possibility is that because there is a reason for making the shehecheyahu here, at the entry of the kedushah of the day, it would not constitute a hefsek, as we see that the shehechiyanu on the second day of Yom Tov does not constitute a hefsek in Kiddush, even though we bring a new fruit to which the shehechiyanu applies (because of the opinion that there is no blessing of shehechiyanu on the second day). Because there is a need and a reason for the blessing, it would still not be a hefsek.
It is noteworthy that virtually all poskim who discuss the matter, including Yecheveh Daas, Iggros Moshe, and others, focus their attention on the problem of women saying amen in kiddush to the shehecheyanu blessing, and do not devote much attention to the question of the hefsek between the blessing on the candles and their lighting. It is presumably assumed that the shehechiyanu blessing is not a hefsek.