Eloping/ Micro wedding! Guest list etiquette? Invite the parents?

Hello everyone! My name is Sabrina and I am getting married! Wahoo! I think I will get down to the point. My fiance and I are pretty minimalistic and private people. We are introverts as you can imagine and I want our wedding to be a joyous UNSTRESSFUL thing. My dilemma is that my parents are you guessed it; separated. And have been since my birth. I was raised primarily by my mom and my older sister. My sister and I are still very close and, well truly best friends. Now, my mom is married to my stepfather whom I am not particularly close and they have 2 children together. My father is also remarried to my stepmother and they also have 3 children together. As you can see the guestlist is getting longer. Now to my second dilemma. My fiance is an orphan. His grandparents who raised him and his older brother have recently passed. So my fiance has really no one to come other than his older brother. I think I would prefer to just invite my older sister and that is it but am afraid my parents and other siblings will be upset with me. I am also afraid that if I just invite my sister then my mom will feel bad and left out. So my question is do I invite my parents and all of my siblings? Do I invite my mom and sister and not invite anyone else? Do I just invite my older sister and hope my parents and other siblings will not be upset? Thank you for any advice you can give me!