Is there a problem with lending my car for the next few weeks to my sister who grew up keeping shabbos but no longer does? Should I ask her not to use it on shabbos and Yom Tov or just not ask? She’s a paramedic anyway so she may have legitimate reasons to use it on shabbos anyway.
It is permitted to lend her the car. There are a few contributing factors why it isn’t lifnei iver to lend her the car even though it is possible that she will drive it on Shabbos. It isn’t clear that she is going to use the car on Shabbos. That is her choice, if she wants to or not. You are merely giving her a car and she will decide if she chooses to do something forbidden with it. It is better if you can suggest to her that you would rather she only use it for permitted uses and not on Shabbos, however if saying this will cause animosity and ill feeling between her and your family, you don’t have to.
May Hashem help that your sister should eventually do teshuva and return to the rich tradition that she has.
Avoda Zara 15a, Ritva ibid 15b D:H Mi Dami, Meiri, Tevuas Shor 16-23.