Dear sit/ma'am,

Please note that concealment of your sister's mental health is a very strong ground for them seeking separation. However, if you can establish that the illness had subsided and the gravity of the situation was not so intense, that it was not necessary to disclose the information to the in-laws, then you can rightly defend yourself. This may require the opinion of your doctor and medical evidence. Thank you.

You ought to have disclosed your sisters mental condition before marriage 

2) her husband can file for anullment of marriage on grounds of fraud 

3) you can contest legal proceedings filed 

4) seek interim maintenance and alimony from husband 

You have to defend by saying that your family members had informed your sister mental health conditions and medical prescription at the time of marriage engagement and her husband side family members had accepted that too and then took marriage ceremony and given assurance that they will take care of it and we should not worry about it.

Under Section 3 of MUSLIM WOMEN (PROTECTION OF RIGHTS ON MARRIAGE) ACT, 2019 Any pronouncement of talaq by a Muslim husband upon his wife, by words, either spoken or written or in electronic form or in any other manner whatsoever, is void, illegal and punishable offence. There is not threat of such divorce to sister. If they try to harass her you can seek remedy against them under Section  498A for cruelty, Domestic Violence Act, 2005 and 125 Code of Criminal Procedure, 1973 for maintenance.

- Concealment of necessary facts at the time of marriage is a ground for annulment of marriage , hence her husband can approach the court for getting divorce legally. 

- However, if the said disease is curable , then it will not come under the concealment.

- If he files a divorce petition , then you should produce the doctors report with the reply of the petition , hence you should take suggestion from a doctor for the gravity of said diseases. 

- Further, your sister can also compliant against her husband and family members for cruelty , and to claim maintenance from him. 

You can contact me , if further suggestion needed. 

The fact of her mental illness which was material for marriage had been suppressed by you people to her in laws/husband, hence they may file a cheating case against you people for this considering this an an offence. 

They can even dissolve the marriage by pronouncing talaq for this reason.

Actually for women, according to the Muslim Marriage Act, 1939, a woman married under the Muslim law shall be entitled to obtain a decree for the dissolution of her marriage (based on mental health issues) on the following grounds:

that the husband has been insane for a period of 2 years.

But a male can pronounce talaq without assigning any reason also.