What is the way to invite my two atheist sisters, who grew up in a non-Muslim country, to Islam? And will the questioner be sinful if she falls short in that? And are there any French books that can be used to invite them?

You must take the initiative to save the two sisters from misguidance, especially the atheist, by clarifying the truth to them and inviting them to return to what is right. You will receive a great reward for this. Calling to Allah is a communal obligation (fard kifayah), and it may become an individual obligation (fard ayn) if there is no one else to do it. The method of dawah differs between the atheist and the transgressor. For the atheist, evidence for the existence of Allah should be presented. You can use books such as: "Al-Aqeedah fi Allah" by Omar Al-Ashqar, "Physics and the Existence of the Creator" by Ja'far Sheikh Idris, "The Story of Faith" by Nadeem Al-Jisr, "Faith and Life" by Yusuf Al-Qaradawi, "Science Calls to Faith" by Wahiduddin Khan, all the works of Abdul Majeed Al-Zindani, and the website "Al-Kashef." As for the other sister, she should be called by addressing her misconceptions or reminding her of the Afterlife and encouraging her to repent and seek forgiveness. You can benefit from French books introducing Islam such as: "Le Noble Coran," "Le jardin des pieux," and the French Islamweb website.