This is a series of riddles, whose answers all have a relation to each other. Answer the riddles to find the relation.

Riddle 1:

I invade and conquer,
I kill all in my path,
Though I do not kill directly.

People can usually fight off my brothers and cousins when they invade,
But when I come,
I allow them to conquer and kill.

Riddle 2:

Some say I conquer all
And I even have my own holiday.

I am given and received
And I can cause joy or grief.

Everybody wants me
And everybody hates to lose me.

Riddle 3:

I am gigantic
And everybody loves to come see me.

I was the home of the government of the USA,
But now I am the home of the theatre.

Though I'm not famous because of fruit,
People call me an apple.

Riddle 4:

I'm given and received,
Although the people who have to give me don't want to part with me.

I'm only given by people who don't own something,
And I'm usually associated with apartments.