Cousins visiting for two weeks, and our toddler's afternoon nap is the first casualty

My partner's sister and her three kids are here till the end of the month. Lovely to have them, house is full, and the noise peaks exactly at 1.30 when our daughter is supposed to be asleep. Four days in, she's napping 30 minutes instead of two hours and is a wreck by 6. I'd rather not be the one shushing everyone all afternoon. Kya karein? How did you protect a little one's nap with a houseful of guests without making it awkward?