The key point is whether your uncle has actually accepted the inheritance. If he hasn't signed any documents at the notary yet and hasn't used the apartment or car, he can refuse the inheritance within six months from the date it opens. Then the debts won't transfer to him - they'll remain in the estate assets, so to speak, and can be paid off from the value of the property when it's sold.
The problem is that after six months pass, refusing becomes much harder. If the documents have already been issued in his name or he's been managing the property in fact, the court may not accept the refusal without serious grounds. In this case, your uncle becomes responsible for the debts up to the value of the inherited property - that is, the debts won't transfer to your mom, but they'll be paid off through selling the apartment and car.
The advice is simple: if your family has agreed that your uncle doesn't want this inheritance, you need to file a refusal at the notary as soon as possible. Then the property can go to the next in line (your mom or other heirs), and they can decide whether to take it with the debts or refuse it too.