All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Among the most important Islamic objectives is that affection and love should prevail in the Muslim society; this is even more confirmed among those who are related to each other, like the case of the in-laws. Hence, they should close the doors to the devil so that he would not spoil this relationship. Allaah Says (what means): {And tell My servants to say that which is best Indeed, Satan induces [dissension] among them. Indeed Satan is ever, to mankind, a clear enemy.} [Quran 17:53]
Besides, the family of the husband should not behave in a manner that could provoke the wife of their son.
As far as you are concerned, we first advise you to be patient with the family of your husband, and then try to take it easy with their behavior which you described in the question; it might be that it is their love for their son or for their grandson that led them to serve them without intending to provoke you. Indeed, your interpretation of their behavior and feeling angry because of that may make you sorrowful and distressful while you do not need all this especially at this time when you are most in need of rest.
As regards disciplining a child who has not yet reached the age of distinction (7 years), then this is not permissible in principle, it is better to guide him in a manner which he understands.
However, if we presume that you were wrong by beating him for example, then it is acceptable for your sister-in-law to advise you in this regard in a soft and gentle way, but she should not look at you badly as this is not appropriate.
As regards them taking your son away from you at this age, then if you know or predominantly think that he will be harmed, then you may object to their taking him away from you. Nonetheless, it is better to try to convince them in a soft and gentle manner to leave the child with his mother as he is still at an age when he might be most in need of her.
Allaah Knows best.