All perfect praise be to Allah, The Lord of The Worlds, and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
Paying cash for gold is permitted if payment is made on the spot. However, buying and selling gold in installments isn`t permissible for fear of incurring usury{Riba}.
Ubida b. al-Simit (Allah be pleased with him) reported Allah's Messenger (PBUH) as saying :Gold is to be paid for by gold, silver by silver, wheat by wheat, barley by barley, dates by dates, and salt by salt, like for like and equal for equal, payment being made hand to hand. If these classes differ, then sell as you wish if payment is made hand to hand."{Muslim}.
As this Hadith indicates, on the spot/hand to hand payment is a condition in paying cash for gold as well as the impermissibility of delaying any.
In conclusion, you(asker) have to go to the jeweler in order to return the gold set and make a new transaction in the correct form stated earlier, if possible. However, since your sister has given that gold as a present and can`t take it back, then it is imperative that she repents and asks for forgiveness, and we pray that Allah forgives her. And Allah knows best.