My girlfriend is coming to the US to see me in December and if all goes well we are going to get married, if we get married are we able to travel back to her home country in April to her brothers wedding without her being considered as deporting herself?

You haven't said where she's from.
Have you two actually met, in person? I ask because of all the internet scams that involve you sending a woman money to get out of her country. (She never arrives.)

I am not sending any money at all, she has paid for it already. She is from England

What type of visa is she coming in with, Is it a visa to get married ?