gistfile1.cs

void Main()
{
	var root = new FamilyItem
	{
		Name = "A",
		FamilyItems = new List<FamilyItem>
		{
			new FamilyItem
			{
				Name = "B",
				FamilyItems = new List<FamilyItem>
				{
					new FamilyItem(),
					new FamilyItem(),
					new FamilyItem()
				}
			},
			new FamilyItem
			{
				Name = "C",
				FamilyItems = new List<FamilyItem>
				{
					new FamilyItem(),
					new FamilyItem(),
					new FamilyItem(),
					new FamilyItem()
				}
			}
		}
	};
	
	Func<FamilyItem, IEnumerable<FamilyItem>> flatten = null;
	
	flatten = (FamilyItem x) =>
	{
		return x.FamilyItems.SelectMany(p => flatten(p))
			.Concat(new[] { x });
	};
	
	var largestFamily = from family in flatten(root)
			    let count = family.FamilyItems != null ? family.FamilyItems.Count : 0
			    orderby count descending
			    select family;
	
	Console.WriteLine(largestFamily.First().Name);
}

class FamilyItem
{
	public FamilyItem()
	{
		FamilyItems = new List<FamilyItem>();
	}
	
	public string Name { get; set; }
	
	public List<FamilyItem> FamilyItems { get; set; }
}