Super fast explanation, but CAT is capital acquisitions tax. If the house is sold to you at a price that is less then market value, then the difference between the market value and the price you pay is deemed a gift from your mother and uncle to you. So if there is an equal joint ownership between the two of them, half of the difference will be deemed a gift from your mother and half a gift from your uncle. Each gift may be subject to CAT. There are different exemptions levels between family members, the closer the relationship the higher the exemption. Won't get into too much more detail as if this set of circumstances applies to you, it sounds like you could do with getting some more formal advice.
HTH