Many Narcissistic Mothers have strange attitudes towards sex. This isn't part of the official criteria, but the experience of so many DONMs with this issue leads me to know it can be part of the mix.
Maybe it's about lack of boundaries which is a totally narcissistic trait. Or maybe (and probably more likely) there is more going on than just Narcissistic Personality Disorder, such as Histronic Personality Disorder. Whatever it is, it's horrible.
They might be over-sexual and act inappropriately flirty, maybe with their daughters' boyfriends or other inappropriate people. (Flirting with their daughters' boyfriends also invades her boundaries, making sure the daughters know their place in the grand scheme of things.)
Or they might blur boundaries again by giving their daughters too much information re their own sex lives.
This might even blur the boundaries between inappropriate behaviour and sexual abuse. In healthy relationships mothers are physically intimate in many ways with their daughters anyway (such as for hygiene reasons when the daughter is young enough, or trying on clothes in a shop together), so it's easy for inappropriate behaviour to slip in in unhealthy relationships. We have heard examples on the forum that definitely veer into sexual abuse, but of such a subtle kind that the victims didn't even realise that's what it was. Which is, in itself, a form of gaslighting.
I am lucky that my own mother did not fall into any of these extremes. In fairness I think she dealt with sexual issues with me very maturely and very appropriately.
But I know that these issues do cause big problems for daughters who've experienced them, and that is all too many.