You might not differentiate too well with "shi" and "si" pinyin in mandarin if you are not a native speaker. Or maybe you pronounce "she" differently. There shouldn't be a "sh" sound in "xi".
I'm a native speaker; my first language was Mandarin.
On the other hand, I'm a Southerner, so I don't usually distinguish between "sh" and "s" sounds. I can do the 卷舌 thing, though, well enough that Northerners tell me I'm doing it right, and I don't think either Pinyin "shi" or Pinyin "si" sound anything like English "she". And while I don't think Pinyin "xi" sounds exactly like English "she", I think it sounds pretty close.