Please take note -
01. First among all, file a criminal case in against your siblings and the servants under section 406, 415, 420, 506 and 120B of the IPC stating that they have robbed your property and are now threatening you and your family of the dire consequences if you make any hullabaloo.
02. Then file a complaint before the concern US authorities with the help of this registered FIR and get your siblings deported back to India as India and USA have MOU signed in between themselves that they will handover any criminal in against whom cases are pending in their respective countries. Please note that it do not matter if your sibling is now a US citizen and have given up her/his Indian passport and citizenship. He/she will still get deported in India to face trial in India.
03. The moment you are successful in getting the FIR registered in against your sibling and thereafter informing it to the US authorities I am sure your sibling will come to settle out this matter with you. If he/she does then start recording every communication in between you and your family and him/her to produce it before the court of law to prove his/her guilt. But if he/she doesn't then he/she will anyway going to contest the case in the courts of India and will have to prove that as to how these properties which she has very tactfully taken away with himself/herself belongs to him/her.
04. Also please note that any investment or purchase made on the name of parents do not comes under the Benami Transactions so he/she can't get you dragged under benami transaction act case. Please don't get worried on this part.
05. Make sure that you get to collect every proof of your own to prove that whatever is there on the name of your mother has been purchased and created by the money which you have invested. The more the documents you will have with yourself the better your case will be at the end of the day.
Most above, hire a good advocate to represent your case otherwise no matter how many proves you have with yourself you will end up losing your case. It is very important to have a good advocate by your side.
I am sure that if you get to do all the above stated then you will easily win the case and will get what belongs to you.
My best wishes to you.