She can commit her share of the property, but she cannot commit yours without your consent.

From what you have told us, you did consent to the renting of the home to your nephew. You don't mention the amount of rent being paid, so apparently you agreed to that as well. Your objection appears to only be to the term of the lease.

Your best option is to discuss the situation with your nephew and sister to try to come to an agreement. If that is not possible, your nephew would at a minimum be entitled to notice from you to terminate the tenancy to which you agreed:

Quote Quoting Florida Statutes, Sec. 83.57. Termination of tenancy without specific term.
A tenancy without a specific duration, as defined in s. 83.46(2) or (3), may be terminated by either party giving written notice in the manner provided in s. 83.56(4), as follows:

(1) When the tenancy is from year to year, by giving not less than 60 days’ notice prior to the end of any annual period;

(2) When the tenancy is from quarter to quarter, by giving not less than 30 days’ notice prior to the end of any quarterly period;

(3) When the tenancy is from month to month, by giving not less than 15 days’ notice prior to the end of any monthly period; and

(4) When the tenancy is from week to week, by giving not less than 7 days’ notice prior to the end of any weekly period.
So, for example, if you agreed to a month-to-month tenancy and he's paying monthly rent, then you would have to give not less than 15 days’ notice prior to the end of any monthly period. During his continuing tenancy, as you consented to the tenancy, he has the right to the exclusive use and possession of the property. Following the notice period, if he has not vacated, you can file an eviction action -- but there's another wrinkle, which is that as a co-owner of the home your sister can give him permission to continue to use the property. Were she to do that, he would then have the same rights to continuing use and possession as you -- and you would be living in the home with a hostile house mate (and with no rent coming to you).

Perhaps it's time to sell -- either by putting the house on the market, or by finding an agreeable price for which your sister and/or your nephew can buy out your interest in the home; or where you buy out your sister's interest. You can compel the sale and division of proceeds from the sale through a legal action for partition and sale, but in doing so you would still have to deal with the life lease issue (which would deter buyers other than your sister or nephew) and the process is typically expensive.

If you can't work out a resolution with your sister and nephew, I suggest consulting a real estate lawyer. If that becomes necessary, take a copy of the "life lease" with you to the consultation.