Praise be to Allah.
Your wearing the niqaab that covers the face is something that is prescribed in Islam, rather it is obligatory, because of a great deal of evidence that we have explained in the answer to question no. 11774. Your sister’s husband has no right to object to that, rather every believer should rejoice at the spread of modesty and virtue.
There is no sin on you because of his seeing you, because that was not intentional on your part. But he is sinning if he looked at you deliberately, because of the report narrated by Muslim (2159) from Jareer ibn ‘Abd-Allaah (may Allaah be pleased with him) who said: I asked the Messenger of Allaah (peace and blessings of Allaah be upon him) about an accidental glance and he told me to avert my gaze.
Your sister’s husband is not one of the relatives with whom you are commanded to uphold ties of kinship, neither is he a mahram with whom it is permissible to shake hands or take off hijab. Rather he is a stranger (non-mahram) to you, and you must cover in front of him, and it is haraam to shake hands with him or be alone with him. Based on that, you do not have to go to his house or get in touch with him, or sit with him, but if you sit wearing niqaab and with your sister or a mahram of yours present, there is no sin in that, so long as there is no cause for doubt or fear of fitnah.
It says in Fataawa al-Lajnah al-Daa’imah (17/420):
The sister’s husband is not one of a woman’s mahrams, and he is regarded as a stranger to her. It is not permissible for her to uncover her face in front of him, or shake hands with him, or be alone with him, or travel with him. He is like any other non-mahram man. But if she sits with him in the presence of one of her mahrams and whilst observing hijab and covering herself, there is nothing wrong with that. End quote.
See also question no. 40618.
But our advice to you is not to sit with him even if there is a mahram present, so long as he is deliberately trying to see you without hijab and he wants you to sit with him and talk to him.
This should not affect your relationship with your sister, because she is one of the relatives with whom you are obliged to uphold ties of kinship. If those ties can be upheld through phone calls and her visits to you, that is sufficient. If she wants you to visit her, then visit her to put her mind at rest, but choose times when her husband will not be at home.
From the above it is known that you do not have to sit with your sister’s husband when he visits you, even if he wants you to. But if you just greet him and leave, that is fine. What matters is that you adhere to the covering, modesty and chastity with which Allaah has blessed you.
We ask Allaah to guide you.
And Allaah knows best.