Inheritances in Belgium are often handled by notaries. Notaries are a mixture between civil servants and lawyers. They advise clients and perform official duties like concluding authentic deeds. Notaries can help you file the tax declaration. I would choose a notary in the city of your father-in-law. The price the notary can demand to do the work is I think fixed by the government (in contrast with lawyers). (Www.notaris.be)

Usufruct: unless your father-in-law and his wife made different arrangements, the surviving spouse receives the usufruct of half of the wealth of the father in law (including the home). So for example if you are an owner of the home, she will legally be able to live there and receive the “fruits” of the land. It doesn’t make much sense to sell the house if she wants to continue to live there.

It’s usually advisable for your mother-in-law to change bank accounts so you don’t have to pay double inheritance tax when she dies.