In game
LifePlay at the start you create your own characters, so you can create practically your entire family in any configuration you want so parent/parents, siblings, aunts, cousins and also grandparents, unfortunately the game is abandoned but despite that it really has a lot of content.
Also in game
Influencing if you additionally install incest patch which can be found on the discord of this game, you can also create several characters and make a family out of them, but you can only set parent(s) and sibling(s), so if you also want to add grandparents you can at most set them as parents of your parents because unfortunately for now game does not give you the option of selecting grandparents as well.