First of all you just ask the relatives to keep away from your family affairs.

Your relatives cannot interfere in your family affairs.

Next, you assure your mother that you will arrange money to return her investment amount and if possible with interest but request her to give you some time and also you can give her some money at frequent intervals ( keep a note of it in writing), and take care of her properly, ask your wife not to indulge in any quarrel with her for any reason, if possible you appoint a special maid to look after her exclusively.

After that also if she is threatening , then you can ask her to proceed legally which can be challenged. 

You collect her mental health and treatment particulars so that you can defend your interests in the event of any untoward incident.

Legally and morally you are bound to maintain your mother. Under senior citizenship Act she can claim maintenance and residence also. Other relatives donot have any role in it. Provide your mother proper medical treatment including psychologist and take care of her in best possible manner. At this age of her expecting any change in her is not possible so take care of her understanding the circumstances. 

Your mother is suffering from bipolar disorder, which is a mental illness that causes extreme mood swings that include emotional highs (mania or hypomania) and lows (depression) . She may be experiencing symptoms such as irritability, impulsiveness, aggression, delusions, hallucinations, or suicidal thoughts . She may also have difficulty in managing her finances, relationships, and daily activities .

As her son, you have a legal obligation to maintain and support her under the Hindu Adoptions and Maintenance Act, 1956 (HAMA) . This act provides that a Hindu is bound to maintain his or her aged or infirm parents if they are unable to maintain themselves out of their own earnings . The amount of maintenance depends on various factors such as the position and status of the parties, the reasonable wants and claims of the claimant, the value of the claimant’s property and income, and the number of persons entitled to be maintained .

However, you also have a right to protect yourself and your wife from any harassment, defamation, or threats from your mother. You can take legal action against her if she is causing you any harm or injury. You can file a complaint with the police or the court under various laws such as the Protection of Women from Domestic Violence Act, 2005 (PWDVA), the Indian Penal Code, 1860 (IPC), or the Mental Health Care Act, 2017 (MHCA) .

The PWDVA provides protection to women from any form of domestic violence by any relative of the husband or the male partner . It also empowers the court to pass various orders such as protection orders, residence orders, monetary relief orders, custody orders, or compensation orders in favour of the aggrieved woman . You can file an application under this act on behalf of your wife if your mother is abusing her physically, mentally, emotionally, or economically .

The IPC defines various offences such as criminal intimidation, defamation, assault, hurt, wrongful restraint, wrongful confinement, etc. that can be committed by any person against another person . You can file a criminal case against your mother if she is committing any of these offences against you or your wife .

The MHCA provides for the rights and care of persons with mental illness and their access to mental health services . It also provides for the procedure for admission and discharge of persons with mental illness in mental health establishments . You can file an application under this act for the assessment and treatment of your mother if she is suffering from a severe mental disorder that requires immediate intervention .

Mother should take her medication daily 

take a loan and repay your mother Rs 16 lakhs given by her 

This is a family issue, and I do not see any legal ramifications of this affecting you.

Although your mother is having various Legal remedies against you and your wife, such as one under the Senior Citizen Act and  DV Act, I don't see her taking recourse to these remedies as you yourself say she is suffering from bipolar disorder. If any time she takes recourse to any legal measure, you may prove her metal illness in the Court and get exonerated. 

- As per law, she is entitled to get maintenance from you , and she can claim the same after filing a case before the court. 

- Further, if she has invested the amount for purchase the flat , then she can claim ownership after submitting the proof of the fund 

- Further, if the said property is in her name , then she can get vacated you and your wife legally . 

- However, if she is mentally ill, then you should take proper care and attention from the doctors , so that she may feel comfortable. 

Dear Client
Consult with a Mental Health Professional:
Consider involving a mental health professional to assess your mother's condition and provide expert opinions on her capacity to make decisions and her behavior. This can help establish whether her actions are a result of her bipolar disorder.

Gather Evidence:
Document any incidents of harassment, defamation, or threats. Keep records of text messages, emails, or any other communication that can be used as evidence. If there were witnesses to these incidents, consider asking them for statements.

Seek Legal Advice:
Consult with a lawyer who specializes in family law and mental health issues. Share all the evidence you've collected and provide a detailed account of the situation. They can advise you on the best legal steps to take.

Restraining Order:
If your mother's behavior poses an immediate threat to you or your wife's safety, your attorney may help you obtain a restraining order or protective order to limit her contact with you.

Guardianship or Conservatorship:
In some cases, if your mother's bipolar disorder significantly impairs her ability to make rational decisions, you may need to consider pursuing guardianship or conservatorship through the court. This would legally designate someone (possibly you) as responsible for her care and financial decisions.

Mediation or Counseling:
Depending on the circumstances and your mother's willingness, mediation or family counseling might be an option to resolve the family dispute and address her concerns.

Financial Matters:
Ensure that you have records of all financial transactions involving the money she gave you for the flat and her maintenance. If these were loans or gifts, make sure you have documentation to support your position.

Stay Calm and Document Everything:
Continue to maintain your composure and document any new incidents or developments as they occur. This will be essential for your legal case.

Consult with Your Local Authorities:
If relatives threaten to involve the police or take matters to court, make sure you consult your lawyer before taking any action. Your attorney can guide you on how to handle such situations.

She can claim maintenance from you legally but can’t harass you