Hi BPDstinks;
Yes, it can definitely happen that there can be multiple pwBPD in a family structure. In my case, my former T suspected that my mom had traits in the past. My high school best friend's mom very obviously had some kind of PD, likely BPD though maybe HPD, and was low functioning. My H's mom used to have many traits, and two of his sisters also have traits, one more strongly than the other. Oh, and the reason I'm here is that my H's kids' mom has many strong BPD traits, and is married to a guy with many NPD traits.
For me, I think that my FOO (family of origin) "primed" me to experience mildly BPD-type dynamics as "normal". So, when I met my H, because he'd also been raised in a FOO permeated with BPD dynamics, he felt "familiar" to me -- the vibes were like, yeah, this is what family feels like.
Marriage and partnership are one way that a family structure can seem to have many pwBPD -- if my FOO has someone with BPD, then I may have a higher chance of partnering with someone who also has BPD in the family. So it can seem to "double".
...
Am I tracking with you that your GD's mom (who is not your daughter) has a mom who is reaching out to you? And you don't suspect
she has BPD, it's just that she's asking for help with GD's mom, who might have BPD?
What kind of advice is she asking for?
How do you usually respond when she asks?
This is a great opportunity to practice your boundaries and communication skills that you've learned here

If I'm understanding your situation, it's something like: it's stressful to you, when your plate is full, that someone well-meaning is leaning on you for help. You just don't have the bandwidth! Maybe you worry that it would be an unpleasant truth to her if you didn't help?
The S.E.T. communication tool is perfect for lovingly sharing an unpleasant truth that you feel you must say.
Support: I would love to help you in this difficult time; I really understand what it's like.
Empathy: You must be heartbroken to be going through this.
Truth: Even though I can't help with personal advice due to my own limitations, I want to share ____________________ resource with you, so you have it to lean on.
What do you think? Up for practicing SET in real life?
(
the B.I.F.F. technique for communications could also help)