I would have loved it if my step mom years ago was like you Panda39, honestly its great.
I think you should just be a supportive, consistent adult who can be trusted.
support this wholeheartedly. Also I would have appreciated if someone could have pointed out what was going on (don't think my dad knew the whole impact) I would and should have been speaking with professionals for support. It has taken me years to get to the bottom of this. I think if my dad would have said I don't know whats going on with your mother but I don't think it would do any harm talking to someone about it ... . then maybe along coming with me (maybe just wait outside). That would have saved me going to the next few years of hell.
You see children from a PD background sometimes lack certain skills for the world and other nasty people captialize on this. The adult child is unprepared, lack of protecting there own boundaries scared of sticking up of themselves (weirdly you wouldn't have thought this if you knew me then, I was both assertative/confident, teachers thought I had it sewn up (but shy at times also!), naive of certain adults with other PD's maybe even different from they're parents. My mom had uBPD/NPD (or thereabouts) but I have had runs in with NPD & ASPD (also BPD dating these before). They sense it somehow... . they like taking the good guys down. Most hideous adult experiences with NPD and ASPD. These are the ones that fill up most of the prisons. (should say out there, not that all NPD/ASPD are criminals). Just most of the criminals will have one of these disorders. Think I'm right in saying that.
The quicker they learn their back ground and skills they may or may not need to master the better for them. I said to my divorced dad I can't understand what 'she' is like, I feel like I'm going nuts... . that would have been a good opportunity. To clarify not that I was going 'nuts' but my mom was and I couldn't work out why she would just drop me for a new partner/life (she divorced my step dad at the time). This will take a lot of trust between the dad and his girls, bit of a balancing act, but he may be able to spot a good opportunity. I wish I was educated a lot sooner because I knew something wasn't right but didn't know the answers, obviously like what Levi78 said... has to be through the dad don't get yourself caught up in it.
Best of luck, you sound lovely, lucky girls
additional thought: If they are empathetic girls - get dad to look into co dependency and how to form decent boundaries. Educate on PD's in general. How to spot a Narc, they are loads on you tube about this. Look out of them on shopping trips or run in with them, make an example. If they are kind girls and have had a PD background their is a chance that PD's will notice. Stuff on assertive skills, learn martial art. Not that that always helps because Narc take the long road sometimes but if they feel empowered by knowing they could look after themselves they will hopefully carry themselves with a level of confidence in public about their every day business, it certainly wouldn't do any harm. Boundaries like, mental, physical, spiritual. Get them to think about how they feel about boundaries, what they're boundaries mean to them etc. Making sure to use language that isn't invalidating, easy to not get right. So they can breath so they are truly heard. What their values are, how they may differ form other members of the family and thats ok too.