Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
First of all, we ask Allah to increase your zeal in doing good and to make us and you remain firm and steadfast on his religion until we meet Him.
Indeed you did well by abiding by the Shariah requirements in dealing with your sisters in-law, as they are alien to you. There is no doubt that it is not permissible to speak with an alien woman unless the religious requirements are met, like wearing Hijab for the woman, not being in seclusion with her and the like.
Talking to women always leads the man to be attracted to them especially nowadays: That is why it is more appropriate to stay away from this except in case of need.
Therefore, you behaved appropriately and in accordance with the Shariah. May Allah reward you and them for that.
So, do not take into consideration those who tell you that this is extremism just to criticize you.
This is not extremism, but it is abiding by the Shariah.
We advise the people who criticize you, and especially your brother in-law, to fear Allah and to help others in being steadfast on the religion of Allah instead of blaming and criticizing them.
Allah knows best.