What is the ruling in Islamic law regarding the limits of my relations with my sisters in-law (my wife's sisters)? Can I sit with them in same room, talk, joke with them, eat with them, take them school in my car as long as they wear Hijab? Are they considered as my real sisters?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The women who become unmarriageable for a temporary reason (such as certain relatives by marriage) are not like the women who are unmarriageable forever (blood relatives, parents of one's spouse, etc.).

Therefore, looking at temporary unmarriageable women or being alone with them is forbidden. They are just like other non-Mahram women as far as the matter of looking at them and mixing with them is concerned, because they are forbidden in marriage only for a temporary reason and they become marriageable as soon as the reason is removed. For example, having two sisters as wives at same time or marrying a woman and her aunt, etc, but when marriage to either one of them is dissolved, it is permissible to marry the other.

Allah Says in the context of the women who are forbidden for marriage (what means): {…and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allah is ever Forgiving and Merciful.} [Quran 4: 23]

Therefore, avoid being alone or mixing with the sisters of your wife since it is forbidden and results in bad consequences.

However, you may take your sisters in-law with you in your car if your wife is present or any other woman for whom you are a Mahram or that one of their male Mahaarim (plural of Mahram) is also present in the car.

Allah knows best.