I agree with the first answer. Everything can be divided equitably in a separation agreement that is binding upon both parties, however, one thing to remember is that after a year of living apart with an agreement in place, either side can move for a divorce and the other side cannot do much about it except to say they didn't live according to the terms of the agreement.
New York law must be different than Minnesota law.
We do not have specific separation agreements. A contract between a married couple is considered a post-nuptial agreement and there are specific factors that must be followed for the agreement to be enforced (including both parties having an attorney and a divorce not starting within a certain time frame after).
Here in MN, it is possible to get a legal separation. To obtain a legal separation, you have to go through a court process nearly the same as a divorce. It is very rare, many legal separations are quickly end up as divorces. A legal separation will determine use and possession of marital assets and responsibility for marital debts and support payments. But it will not result in financial independence for your sister.
Your sister should consult with a divorce attorney so that she can gain a better understanding of divorce and legal separation and understand what could or could not give her the protection she is looking for.
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