What is going on with my gf?

My gf and i work together together.We been together for 2 year and no one at work know we bf and gf which we agree will be best.Bvt why won't she tell her relatives and sister that we are bf and gf? I only met her sister and her sister think we only friends.The sex with my gf is great.I told her i love her.She told me she want to marry me someday.in public she don't kiss me or hug me.She said she don't like to do that in pvblic.She live with her sister and they moved into her aunt house.My gf never ask me over or give me her address.We only have sex at my place.She say she don't have facebook or email.I know for svre she live with her sister.She have mood swings like one day she yell at me and said we are over.then the next day she apolise.And another time she had her phone number disconnected and a few days later she gave me her new phone number.Lately she always have excuses why she can't go out.Like she have to do something with her family or going to the dr or tired from work.Since we work together i know it true.The time we went out for brunch her sister went along.We talk on the phone and text.Do you think her sister know we more than friends?I buy her gifts but she never buy me anything.She said a man should support a woman.How can i get her to spend more time with me?She tell me she my gf.Why won't she tell anyone we bf and gf?