Should I tell my parents?

My sister was seeing a 34 year old man and she's 17 my mum and dad weren't fine with it but they put up with it because she threatened to run away, in the end everyone found out he was keeping my sister a secret and seeing his wife, I know my sister still she's him like twice a week but she don't know I know, I've found evidence and stuff, and now I just found a pregnancy test (she's not pregnant) but the whole time she's said she hasn't slept with him (since April) but why would she do a test, I don't know wether to tell my parents she's still seeing him and prove by the pregnancy test or should I leave it? Help ASAP please