Dear tired mom, I hope you you make some time for peace in your day today.
I might be inclined to let it go. If it was a life-threatening concern or something related to health or safety, I'm sure your cousin would have shared the information. My best guess is that it was just enough info to bait someone into contacting you? Does that sound right? My pwBPD is an expert at this type of baiting.
There's not always a right or wrong so trust your gut, of course, and keep your boundaries in place.