Alumium Wiring in Charlotte, NC
Aluminum is no longer used in house wiring anywhere in the USA where the NFPA Electrical Codes are accepted that I know of, it had been used many years ago and is as far as I know against code (some one might have a guide to the NFPA NEC that states this). Your home might have been built before the new codes were adopted - so it exists as is. 

Of course that really depends on state and county and when they adopted the codes; you should talk to the building code department of your state and county to verify when the change took place and when your house was built. Further take note of ANY electrical upgrades that were done, any building modifications or renovations - the reason being is that there are legal remedies you might have against the seller to get the wiring replaced. It might be considered a material issue (as in facts) that is required for disclosure ..for this you would need to consult a real estate attorney in your area.

The problem with aluminum was a thing called Galvanic Corrosion - it created a very high potential for electrical fires corrosion would occur and the connection would break loose and then you would get sparks etc and create flames, which is why it was removed from use, although I think in some cases they allowed it to continue to be used for ground wire - but DON'T quote me on that.

Now on to your questions:
Pig Tailing seems to be allowed and is one method to fix the issue with Aluminum wiring with out replacing all of the wiring in the home .

See this link for more details: Charlotte NC Aluminum Wiring Repair