I feel like I cant have my own space sometimes?

Explain to your sis that you are getting older and while you do not want to make her feel like she is being left out, you have to be able to divide your time as well as your schedule will permit. Try taking your sister out with you and your gf to the movies or just walking around the mall or something she enjoys doing. Tell her that you did have more time to hang out before but now you have someone you have to show how important and specail she is to you so that you do not lose her. Ask your sis if she wants you to be happy and of course she will say yes. Tell her in order for you to be happy you would like to be able to spend time with your gf without making sis feel bad. Tell her you apologize if you have made her feel left out and you will try to set aside a day or two out of the week just to hang out with your sis so this can be avoided. Let her pick a day to hang out so that she has a choice in the decision making. makeing people feel like its partly their idea makes them feel a lot better.