My cousin sister asks me things that I don't want to share, yet I share details with her because I don't know a polite way to tell her that I don't want to share. 

From time to time, she asks me about my salary and how much I am saving. I don't like to share my salary and savings details, yet I tell her when she asks and regret later. 

Once in a while, she would ask me about other family members and I happen to share some details, though I don't want to gossip like this. 

A few weeks ago, she asked me what gift I gave to my niece. I had no problem sharing this, so, I shared this. But then she asked how much that gift cost me. Even though I didn't want to share the price, I divulged it because I didn't want to offend her by saying that the information she was seeking was private to me. 

I would like to add that invariably she would start the conversations warmly and revolving around me only, so, I would feel like I was in a safe place and then she would ask something private. At that point, I won't know how to back off. 

Also, I call her only on festivals to wish and not otherwise. But she calls pretty often. Lately, I have stopped picking up her calls because I realised I was behaving differently around her than I expected of myself. But then she would text me to call her. And I feel obliged. Once she kept calling at 11pm and when I gave in out of worry, I found out she just needed my netflix password (again, a private info)!