Is it a sin if one does not do the ‘aqeeqah or delays it?
Question

Allaah has blessed me with a baby girl and she is now three months old, but I did not slaughter the ‘aqeeqah or give charity in its stead. Is there any sin on me, and what is the solution?.
Praise be to Allah.The ‘aqeeqah is Sunnah mu’akkadah (a confirmed Sunnah), and there is no sin on the one who does not do it. That is because of the report narrated by Abu Dawood (2842) from ‘Amr ibn Shu’ayb from his father from his grandfather who said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Whoever has a child born to him, and wants to offer a sacrifice on his behalf, let him do so, for a boy two sheep that meet the conditions of sacrifice and for a girl one sheep.” This hadeeth was classed as hasan by al-Albaani in Saheeh Abi Dawood. 
The Prophet (peace and blessings of Allaah be upon him) left the matter up to the wishes of the person doing it. This indicates that it is mustahabb and not obligatory. 
See Tuhfat al-Mawdood, p. 157. 
But the Muslim should not be negligent in this matter, because the Prophet (peace and blessings of Allaah be upon him) said: “Every child is in pledge for his ‘aqeeqah which should be sacrificed for him on the seventh day, and his head should be shaved and he should be given a name.”
Narrated by al-Nasaa’i, 4220; Abu Dawood, 2838; al-Tirmidhi, 1522; Ibn Maajah, 3165; classed as saheeh by al-Albaani in Saheeh Abi Dawood. 
So you should do the ‘aqeeqah now, by slaughtering a sheep with the intention of ‘aqeeqah. 
In says in Fataawa al-Lajnah al-Daa’imah (11/934): 
‘Aqeeqah is Sunnah mu’akkadah, for a boy two sheep which are both fit for sacrifice, and for a girl one sheep, which should be slaughtered on the seventh day. If it is delayed until after the seventh day, it is permissible to slaughter it at any time, and there is no sin in delaying it, but it is better to do it sooner if possible. 
Note: 
You say that you have not given anything in charity instead of the ‘aqeeqah. 
It should be noted that giving money in charity cannot take the place of the ‘aqeeqah, because the purpose of the ‘aqeeqah is to draw closer to Allaah by offering a sacrifice. See question no. 34974 . 
And Allaah knows best.