To solve this problem, we compute the total number of guests summing the number of guests of each group. The total number of guests is 12 + 6 + 4 + 2 = 24. A 75% of the guests is equal to 0.75*24 = 18 guests. Summing the 12 cousins, the 4 brothers and the 2 sisters, we get 18 guests, so those groups represent 75% of the guests.
Answer
The 12 cousins, 4 brothers and the 2 sisters represent 75% of the guests.