what should i do? should i keep talkin to the girl that i try to help or listin to my girlfriend wishes?

my girlfriend sierra dosent like when alot of girls be around me all the time. but there is this one girl named crystal that i give alot of advice to about her life and things that she should do to be on the right track. but sierra says most of the time when she see me she see crystal to. and in a way she dosent like that because she think crystal has some feelings for me cause of the fact that me and her is so close, and in the past sierra told me that she was a hoe and use to mess with alot of dudes. and now every time she see me with her she allways have something 2 say. so what should i do??? should i stop talkin to crystal????