All perfect praise be to Allah, The Lord of the Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
In principle, the parents or guardians of a child are responsible for making his Aqeeqah , but if they didn`t and he/she reached the age of puberty they become free from liability.
After reaching puberty, it is recommended that a person makes Aqeeqah on behalf of himself/herself, and it doesn`t avail that someone else offers it on his/her behalf since Muslim jurists haven`t permitted offering Aqiqa on behalf of an adult. Al-Imam An-Nawawi (May Allah be pleased with him) said: "None is permitted to offer Aqeeqah on behalf of an adult but himself/herself." {Al-Majmou`, 8/431}.
This because, in principle, acts of devotion should be performed by the person himself. However, if someone else offered Aqeeqah on his behalf after the latter had reached the age of puberty then both will receive the reward of charity and that of slaughtering a sheep for the sake of Allah, but it doesn`t avail as Aqeeqah .
In your case (Questioner), you should give that sheep to your sister, and after it becomes in her possession she may authorize someone else to slaughter it as Aqeeqah on her behalf. And Allah knows best.