After Pip's first meeting with Miss Havisham, he is ashamed of his circumstances, of his family, and of his own rough, ungentlemanly ways. Before he went there, he narrates that he was constantly frustrated with the way that his family and their friends teased him, analyzed him, insulted him, and took every aspect of his life and hashed it out in detail, making fun of it and making it seem like it was worthless. The greatest instigator of these conversations were the insulting comments of Uncle Pumblechook, whom Pip's sister deferred to in all things because of his slightly higher social rank. He would come over and humiliate Pip constantly, and Pip's sister would let it happen out of respect for his rank.
So, when his sister asks about Miss Havisham, he doesn't want his special experience that had so personally altered him to be out there in the open, to be openly mocked. Also, they had heard such fantastical rumors about Miss Havisham, that he felt that the truth, the very sad truth of her state, would be a disappointment to them. So, lying and exaggerating was easier, and got them off of his back for a while. If he could get them going on some tangent, it meant that at least they weren't questioning him, and he could be left alone with his thoughts of inferiority, and to process the experiences that really occurred. He wanted to keep his world at home separate from the strange and dazzling world at Miss Havisham's. I hope that those thoughts helped; good luck!
In my opinion, Pip lied about Miss Havisham because she is way too bizarre. For example, Mr. Pumblechook and Mrs. Joe believes that Miss Havisham is one whom is very elegant and sophisticated living the elegant life. Of course though, in reality Miss Havisham and her house is weird and extremely creepy. If Pip tells them the truth, Mrs Joe and Mr Pumblechook would think that Pip is lying to them and a whole new problem would happen to Pip.
Also, I think that Pip might just wanted to keep Miss Havisham's life style to himself. He did not want to talk behind her back, and tell them how horrible of a person she is.
In Ch.9 Pip returns home from his first visit to Satis House. His mind is still in a state of shock and he hasn't yet fully recovered from the shocking sight of the bizarre Miss Havisham and the humiliating treatment meted out to him by Estella. He knows that he will soon be interrogated by all the adults and that he would have to satisfy especially the curiosity of his sister. But with great presence of mind he decides not to tell the truth and he explains to the readers the reason for his decision thus:
I felt convinced that if I described Miss Havisham's as my eyes had seen it, I should not be understood. Not only that, but I felt convinced that Miss Havisham too would not be understood; and although she was perfectly incomprehensible to me, I entertained an impression that there would be something coarse and treacherous in my dragging her as she really was (to say nothing of Miss Estella) before the contemplation of Mrs. Joe. Consequently, I said as little as I could, and had my face shoved against the kitchen wall.
Thus, we see that there are four reasons for Pip to lie to his sister:
1. "I should not be understood." Pip was convinced that what he had seen was so grotesque that his sister would not believe him if he described Miss Havisham exactly as he had seen her.
2." she was perfectly incomprehensible to me." Pip being a small boy had never seen anything or anyone resembling Miss Havisham. As he tells us very plainly he himself could not understand the weird appearance and odd behavior of Miss Havisham, so how would it be possible for him to give a true description of her to his sister?
3. "there would be something coarse and treacherous in my dragging her as she really was (to say nothing of Miss Estella) before the contemplation of Mrs. Joe." Although pip had been terribly scared by what he had seen of Miss Havisham and had experienced at the hands of Estella it had been a uniquely personal experience. He did not wish to trivialize this extraordinary experience by discussing it with his sister.
4. "a dread of not being understood," Miss Havisham was such a strange creature that Pip was frightened to describe her exactly as he had seen her, because he was convinced that no one would believe him if he did so.