Welcome. First thing to do is to let your cousin know you want to host a baby shower for her and give her an idea of how many people you can accomodate. That way she can decide whether it will be fore her family, her DH's/SO's family, friends or all of them. Personally, I had 3 showers - one that was family and close friends, one that was given by the women at my church and a work shower. If you have a budget in mind that might determine how many you can accomodate space-wise and financially.
I get a lot of ideas from pinterest and also google.