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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
First of all, we would like to stress that the in-laws should be keen on having good relations and should be affectionate towards each other. This is required between the Muslims in general, but it is more emphatic between people who are related to each other through marriage and the like. Indeed, in the Prophet, sallallaahu ‘alayhi wa sallam, there is a good example for all of us, as he, sallallaahu ‘alayhi wa sallam, always maintained good relations with his in-laws. Allaah says (what means): {There has certainly been for you in the Messenger of Allaah an excellent pattern for anyone whose hope is in Allaah and the Last Day and [who] remembers Allaah often.} [Quran 33:21]
The good relationship between the in-laws is one of the reasons of happiness, so it should not be spoiled.
Your husband has no right to prevent you from contacting your sister; this is an order to cut your ties of kinship, so he is not to be obeyed in this regard. Hence, you have the right to communicate with her by phone or internet and so forth, but you should do so without his knowledge in order to avoid problems. As regards visiting your sister, then the scholars differed in opinion about its ruling, but it is better to obey your husband in this regard in order to avoid unpleasant consequences.
On the other hand, your husband threatening you with divorce in such cases is a kind of bad marital relationship. Marital life is too important to be jeopardized or ruined for trivial reasons. Allaah called marriage a solemn covenant; Allaah says (what means): {And how could you take it while you have gone in unto each other and they have taken from you a solemn covenant?} [Quran 4:21]
So it is required to preserve such a covenant.
Also, your husband has no right to prevent you from giving your sister the power of attorney regarding inheritance. This is one of the private matters of a woman, so you are not obliged to obey him in this regard. Some scholars stated that obedience to the husband is related to marriage and matters related to it [marriage] only. For more benefit, please refer to fataawa 89815 and 93972.
Finally, we recommend, in any case, that you be wise in dealing with your husband. You should seek the help of some righteous people to endeavor to mend the rift and reconcile between your husband and your family as this is one of the greatest acts of worship by which one gets closer to Allaah; Allaah says (what means): {No good is there in much of their private conversation, except for those who enjoin charity or that which is right or conciliation between people. And whoever does that seeking means to the approval of Allaah - then We are going to give him a great reward.} [Quran 4:114]
Abu ad-Dardaa’ narrated that the Prophet said, “Should I not tell you about what is better than the degrees of fasting, prayer and charity?” They (the Companions) said, “Yes.” He, sallallaahu ‘alayhi wa sallam, said, “It is to reconcile between people. Spoiling relations between them is al-haaliqah (the shaver, i.e. destructive).” [Abu Daawood]
Allaah knows best.