Your question is difficult to answer, as I don’t completely understand your family structure. You make it sound as though the two daughters you are writing about are not living with the same family all the time (“My 5-year-old is being raised with her twelve sisters and cousins,” whereas nothing like that is said about the younger one). That’s a lot of sisters and cousins, and, whatever else it means, it suggests that the younger children in the family cannot possibly get a lot of attention.
What is most surprising is that the 3-year-old seems to be doing so well. One of our myths is that the youngest children in a big family should be smarter than the other kids because they supposedly get so much help from the bigger ones. That’s not the way it happens though as, during the early years, it is input from the parents that most influences early learning, and youngest children in big families simply do not get as much adult attention as the ones that came earlier in the sequence.
Be that as it may, and whatever your family arrangement, you need to face the reality that we can’t guarantee that all our children will be equally competent. While all loving parents want to equalize their children to some extent, we don’t necessarily want to penalize a brighter child by trying to hold that child down to the level of another in the family. It sounds as though your 3-year-old has a livelier intellect than your 5-year-old. Your challenge, I think, is to find ways to help the 5-year-old without putting a damper on your younger daughter.
Private, individual time with your 5-year-old is important. If she always has to compete with a big group of siblings and cousins who are older and abler, she may have come to expect that she can’t do things and gives up before she really tries. Find some toys she especially likes and sit down and play with her; if the 3-year-old approaches and tries to take over, firmly send her away and say you’ll play with her later. And don’t worry too much about kindergarten homework; many of us consider it an abomination and think it should be outlawed. Find things that give your daughter just a small, manageable challenge. And be sure to praise her when she is successful.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.