"Here baby! Let me help you!" Lil said to Liam. I picture her still calling him this when he's 30...
It only appears here that Lil is strangling her little brother. But she was earnestly trying to assist him here.
With a smile on her face, she appears to be choking him with compassion.
She was trying to assist him out of his little red wagon to save him from toppling over for when he would stand up on his own. Because it is on wheels, it often starts to wiggle back and forth, easily causing the little person attempting to stand from falling over the side of it.
Lily didn't realize it'd be easier - and safer - to simply hold the wagon.