In the recorded phone call, LS said that she had a better relationship with younger sis L than Gannon, because L was younger (2 yo) when she started living with them, so she bonded with T more readily. If I'm not mistaken, lil L had also been living with them a bit longer than Gannon.
IME nannying, 2-4 year-olds are relatively easy to care for. At that age, they rarely actually question a caregiver's authority. They tend to be very attached to their parents/caregivers and often quite affectionate.
School-aged children are different because they're starting to develop a personal identity and as part of that, they frequently challenge parents and other caregivers. It's a normal part of development for school-aged kids to become less attached to caregivers
T probably found lil L much easier to control and more affectionate than Gannon. He probably didn't feed her ego as much as Lil L did.