My grandmother is deceased, during the inventory my mother died and now what?

An inventory is the means by which we transfer the property of a deceased person to their heir. In your mother's case, it's her heir. Since she has also passed away, as soon as the first inventory (your grandmother's) is finished, you will only have to do her inventory (your mother's), precisely so that it passes from her to her heirs, in this case, you and your siblings. In short, you'll need to make two inventories in part so that if you have any real estate, valuables, etc., they can be correctly allocated to your heirs as well, so it's advisable to talk to a lawyer if you already have one.