usually guys do heavier weights with less reps and mostly women do lighter weights with more reps for tone, if you feel capable of increasing the weight of the weights go for it, if your body gets used to those 5 lb weights your work out will be pointless, thats why when you do weight lifting you increase your weights every few weeks to challege your muscles more since they should have become used to the weights you usually use...a rule of thumb for usually is you should be able to do at least 8 reps, if you cant its too heavy, you shouldn't be able to easily do more than 12, if you can..the weight is too light! but I dont know how it goes for men and weight lifting..my boyfriend usually does 10 reps of the heaviest weight he can stand...and does 3 sets...but as for what your sister said, you should do heavier weights..if you are capable..dont stay on the lighter ones or you'll see no growth!

If you are thinking about expense, yes, buying set weights are useless. Assuming you actually use it, eventually you will be too strong to get any benefits out of that specific weight and will need to resort to buying another one later on.

If you buy bar bells however, with variable weights, that may last you quite a while longer depending on what it is you intend to do with it.

No, your sister is incorrect, unless 5 pounds is actually difficult for you to do lets say a bicep curl, you will not "build" muscle (I assume you mean build by getting larger in mass) because you need to cause considerable stress to your muscles to promote growth.