Why isy sister acting this way?

My sister is usually keeping to herself as much as possible. She will not include my brother and I in as many things as possible. She yells at us when we dot meet her expectations, which I hard because she is a wedding cake maker ndm a perfectionist. She is upset because my mon got out of a ba relationship an we ran away into my sisters safe arms. So now we are dependent on her, and I try really hard to make her happy. I give her any money I get, and I try not to eat more than my hate of kitchen food, a bowl of cereal and a sandwich. All of a sudden, she invites us to go shopping with her at target. We go and I find things I like, and she lets us put them in the cart. We didn't even get what we came there for and spent over $200. We went to the movies and watched dispicable me 2, and got popcorn, $36. We get burritos, $21. We go to the movies and eat dinner and watch man of steel, $98. All in a matter of 1 and a half weeks this stuff happens. We don't have this kind of money, and I am crying because I only now realized I am a money eating monster. The only time she yells at us now is when we don't clean up the house. I am scared that she found out that she is going to die and now she is trying to have a good time with my brother and I, or that he went in debt and sort of gave up or something. I am just really scared because I don't know what to expect. A raise in work this dramatic is highly unlikely. I am just really scared of why she is doing this. Has anyone else had a sister like this? What is she doing? Also, all this happened after I had told her I felt like I was a chore to her, and she looked at me really serious and said "I don't want you to feel this way. You are special to me, and I want you to feel that way. I will try to make you feel that way" and now I am really scared. If I let my guard down, like I always do, she wil snap viciously and say she spent all this money on me. If I don't, she wil die on me. What do I do? What's going on?