What is the function of the first Act?

I assume you mean the first scene of Act I, the dinner table scene. The play overall concerns itself with questions of feminism and identity - one of the central enduring questions should be 'what has Marlene given up to become successful in a patriarchal world?' The answer is more than just her child; she has also betrayed the impressiveness of having risen as a woman by continuing to step on others. In the first scene, Marlene confronts these many famous women from history, most of whom progressed as she did - by pushing their individuality to the detriment of others. They are all an aggressive bunch, and so are the right choices to compliment Marlene on her promotion. By setting up these conflicts - a woman in the world is so heavily discriminated against that she must betray her feminism if she wants to transcend it (or put another way, to rise above constraints, she must become her enemy) - the more 'realistic' scenes that follow with Marlene and her sister are viewed in a more harsh light.