Can one sibling rent his apartment before building is resolved and without agreement by other sibling?

The information you provided is not complete. Has a probate been opened for your mother's estate? If so, is the real estate subject to the probate? Is your brother the court appointed estate representative? If the property is part of a probate estate and not yet distributed then your brother would not have the authority to lease the property. If your brother is the estate representative then he can enter into a lease for the benefit of the estate, however, the transaction may be subject to court order. There are risks and liability issues with the estate being a landlord. It also complicates the distribution of the asset. You should collect all the papers you have relating to your mother's estate and meet with an attorney to determine the proper course of action.

There are a number of issues when you are dealing with being a landlord. Typically I structure things where it is set up as a business in a business entity. I may even use another entity to act as the leasing agent or building manager depending on what we want to accomplish with taxes, benefits, exposure to liability, etc. In this case, it sounds like your mom owned a building in her name. Therefore, it is not part of the estate. Selling a building with a tenant could be easier or could be harder. Sometimes the buyer wants a tenant (provided they are paying market price rent ? other times, the purchaser wants to use the property so you have to buy-out the tenant from the lease. I suggest you speak with an attorney regarding the handling of the entire estate. There is likely more going on here than I can see by your description that needs to be addressed.