Life estates don't have beneficiaries, so it's not entirely clear what's happening here.
A life estate gives one person (the life tenant) the right to possess real property for a period of time measured by some person's life. Typically, but not always, the life estate is measured by the life of the life tenant. Once the person by whose life the life estate is mentioned dies, the property reverts to another person (called the remainderman or remaindermen). If the remainderman dies before the life tenant, then the remainder rights typically go to the remainderman's heirs, but it depends on exactly what the deed says. With that said:
I assume what you mean is that someone created a life estate, which I'll assume unless you say otherwise is based on your sister's life, with your sister being the life tenant and her sons being the remaindermen. Correct?
Please explain exactly what this means. Removed from what? And how?
No one who hasn't seen the deed that created the life estate and whatever other documents are relevant can answer this question intelligently. What I can tell you is that your opinion about the "worth" of your two great-nephews is utterly irrelevant to the issue. Also, as explained above, if your sister is the life tenant and the life estate is measured by her life, then the life estate continues regardless of the deaths of the two remaindermen.
I'm not sure what this has to do with the life estate, but if your sister has significant real estate holdings that can or do generate income needed to care for her, then that's great. Has a legal guardian or conservator been appointed to care for your dementia-stricken sister and manager her assets? If not, and if you're interesting in undertaking to do that, then I'd suggest that you consult with a local attorney for advice.

