If your mother took out a reverse mortgage on a home that she owns in her own name, you and your siblings have no obligations.
You don't say whether your
mother's home is partially owned by any of her children (she wouldn't have been able to get a reverse mortgage if any of the homeowners on the title were under the age of 65.) If you or your siblings are on the title, you would have been required to sign off on the reverse mortgage as a co-applicant. In that case, you would be legally obligated by the terms of the loan.
If your mother sells her home or if the home is passed on to you or your siblings when your mother dies, you will then be faced with the task of paying back the reverse mortgage. You can do this either by selling your mother's house or taking out a traditional mortgage and paying off the reverse mortgage with the proceeds of that loan.