15 minutes each to say hi to someone (and likely do a mini catch-up, since it seems they rarely talk) does not seem very long to me at all. Given that it's a holiday, I think it is completely fine. If she was hosting your on a different day, I would think she'd relay to cousin that she had company and would call back.
But this is really common in my family, to take calls from an out of state relative on holidays, talk, pass the phone around, etc. I get that you aren't family in this situation, but you were at their house on a holiday.