Should I believe my sister? Any advice?

[becca]

Alright so basically my sister (15) said that she was pregnant when she said afterwards that she was still a virgin . She's been blanking me since my birthday on Monday, so I messaged her friend on Facebook and she told me that my sister sent her a message that seemed to her that my sister tried to kill herself . My sister said that shes only stressed out because of exams, and says that shes scared of killing herself and she promised me that she wouldn't . She said that she tried to kill herself three weeks ago about a month ago, when I was getting wound up because she was kicking off at me. Me and her friend are getting fed up with the way my sister acts towards us, and i left her a few messages for her to blank me and talk to her friend on the same app instead. It seems as though she is lying and i dont know what to believe anymore. Sorry if this seems mumbled up, its because im worried about her as someone who has tried to kill themselves before. Would you believe this ? Any advice?