Mm,
I'm sorry to know your plight.
In my view, you need not worry as you got every legal rights to claim your husband's benefits (I presume your husband is no more) Legally the nomination given prior to the marriage of an employee becomes void after his/her marriage. You will read more about it in the article attached which will give some more hope to you.
Notwithstanding your legal rights, I have a few suggestions to you,which I mention herein considering the threats you are facing from your in-laws. This purely with a view to avoid all kinds of litigations you may face from them and enable you save some time and money involved in such disputes:
1. With the help of your elders in your family circle try to impress upon your in-laws your legal rights and agree for an amicable settlement acceptable to you.
2. Immediate:
i.Send notices to all the concerned officials stating that - you are the legal wife/heir and also acting on behalf of your children and all the benefits/payments relating to your husband should only be settled to you and they shouldn't act on the basis of the old nominations filed by your husband prior to your marriage.
ii.File claim forms (in the prescribed format) with the concerned officials for PF, Gratuity, leave salary, salary dues if any, Insurance etc. and carefully obtain acknowledgement of filing these claims.
2. As a precaution, immediately file a petition in appropriate court and obtain a stay/ injunction against any payment of your hubby's benefits. This will avert any danger of early disbursement based on these nominations.
3. Also file petition to obtain "succession/legal heir certificate" in your favour (this should also include your children including minors, you as guardian).
4. You may use the help of an advocate in all these process to avoid any problem.
Wish you all the best.
kumar.s.