Legal Question in Landlord & Tenant Law in Massachusetts

Why should she want to smoke in the apartment anyway?

It has been proven to be unhealthy - and the landlord's insurer may require the clause anyway, as smoking in an apartment substantially increases the risk of fires. Smoking is also very expensive, at perhaps nine dollars for a package of 20, and if one is on section 8, one is probably short on resources.

You have not provided enough information for me to answer this question, as the timing of this request, and your sister's prior relationship with the landlord matter; but I suggest she consider giving up smoking in the apartment for many other good reasons.