I stand by my assertion, if everyone who works in the fish store I'd be amazed. Amazed in the same way if someone told me every bagger at my local grocery store was an industrial engineer. If they are, wow, incredible find, but I'm not expecting to find a four year degreed researcher netting fish at a local fish shop. Now if we're talking every adult in the shop, maybe just the shop owner I can believe that.
If someone gives you incredibly bad advice you have to question their knowledge on the subject. If the guys at the LFS really did tell your sister that it is perfectly fine to keep two fancy goldfish, a rainbow shark, and a pair of guppies in the same 10 gallon tank they're either incompetent or just trying to sell her as many fish as possible. Those are your options. No one who is giving out good advice or concerned about her keeping fish successfully would tell her that set up will work.
1) Cold water fish and tropicals in the same tank.
2) Not one but two gold fish in a ten gallon tank that would barely be big enough for a single goldfish on its own.
3) A rainbow shark in too small a tank guaranteeing problems when the shark grows as it gets territorial and starts to fight with the other fish.
4) Male guppy with a single female guppy. The male will harass the female and stress her and if his attentions aren't lethal to her they'll start to make lots of little guppies.
Four very bad, very wrong things with the set up that the guy at the LFS said was fine. I'm not saying he's not a great guy that he's not a wonderful person but if that's the kind of advice he's giving then you need to take anything he says with a serious grain of salt.