You should not be insecure You have a valid ST certificate

you have been brought up in a tribal area your mother is tribal and father also had tribal certificate and your family basically consist  of tribals 

Your father’s ST certificate, issued under the 1977 MHA circular, is valid if properly scrutinized. As he lived in the tribal community and the certificate has been in use for decades, nullification is unlikely unless fraud is proven.

What to Do:

1. Keep all supporting documents (ST certificate, tribal residence proof).
2. Respond promptly if questioned with valid evidence.
3. Consult a legal expert if needed.

The risk of nullification is low if the certificate was legitimately issued.

For personalized advice, consider a phone consultancy. Hope this helps! Spare two minutes for a review—much appreciated! Thank you. Shubham Goyal.

There's no illegality in your father obtaining ST certificate based on his mother's community.

His mother belonged to ST community hence he had an option to choose the community of either of his parents.

Therefore his option to choose his mother's community is very legally valid and cannot be disputed by anyone.

The people who are questioning the legitimacy of his ST community are ignorant of law.

This s the settled law that case status of a child may be determined by the caste of his mother than his father if the child is raised by the mother alone in her own caste and tribe.

In this case the mother did not sacrifice her caste and rather her husband the same caste and culture.

Therefore, there is nothing wrong in the ST Certificate. 

It depends on what the referred circular MHA 1977 says. There are many circulars issued in 1977. Refer to exact details of circular on which you rely. 

The paternal lineage will be taken as the authentic and settled one to determine the offspring's caste. However, if the child's mother belongs to a reserved category and the child wish to get benefits, in view of her mother's backward caste, it's possible. The child can choose the community of either parent.

The caste of a child is usually determined by the caste of their parents, especially the father. 
However, the caste of a child can vary depending on state laws, government policies, and court judgments. 
In some cases, a child can choose to belong to either parent's caste. 
A child can also get a community certificate that lists the caste of either parent. 

ST certificate would not be nullified 

These cases are very common in various Courts of India and Supreme Court. I can understand your concern in this regard. I have examined the link you posted and annexure B regarding the legal views on the issue. 

As you know that scheduled Tribes are the people belonging to tribes, who often live in forests, although there are some nomadic tribes as well. As these people are usually not a part of any organized religion, they are commonly considered outcastes. Socially they have their own traditions, dressing styles, food habits and a distinguished culture.

In State of Maharashtra v. Milind (2001) Hon'ble Supreme Court held that by virtue of powers vested under Articles 341 and 342 of the Constitution of India, the President is empowered to issue public notification for the first time specifying the castes, races or tribes or part of or groups within castes, races, or tribes which shall, for the purposes of the Constitution be deemed to be Scheduled Castes or Scheduled Tribes in relation to a State or Union Territory, as the case may be. The language and terms of Articles 341 and 342 are identical. What is said in relation to Article 341 mutatis mutandis applies to Article 342. The laudable object of the said articles is to
provide additional protection to the members of the Scheduled Castes and Scheduled Tribes having regard to social and educational backwardness from which they have been suffering since a considerable length of time. The words “castes” or “tribes” in the expression “Scheduled Castes” and “Scheduled Tribes” are not used in the ordinary sense of the terms but are used in the sense of the definitions contained in Articles 366(24) and 366(25). In this view, a caste is a Scheduled Caste or a tribe is a Scheduled Tribe only if they are included in the President's Orders issued under Articles 341 and 342 for the purpose of the Constitution. Exercising the powers vested in him, the President has issued the Constitution (Scheduled Castes) Order, 1950 and the Constitution (Scheduled Tribes) Order, 1950. Subsequently, some orders were issued under the said articles in relation to Union Territories and other States and there have been certain amendments in relation to Orders issued, by
amendment Acts passed by Parliament.

Subsequently in KP Manu v. Chairman, Scrutiny Committee (2015) Hon'ble Supreme Court held that three things that need to be established by a person who claims to be a beneficiary of the caste certificate are:(i) there must be absolutely clear-cut proof that he belongs to the caste that has been recognized by the Constitution (Scheduled Castes) Order,1950; (ii) there has been reconversion to the original religion to which the parents and earlier generations had belonged; and (iii)there has to be evidence establishing the acceptance by the community. 

Apparently, in the given facts, you belong to ST. However, detailed discussion is required in such cases with complete facts/documents. 

You may contact my secretary to connect with me for clarification. 

As father was issued ST certificate and he was accepted by your community, your status as ST cannot be nullified as per the circular referred by you. No need to be worried.