Hi Jake - i agree with Emz talking to your Gran will help and having some-way to look at the memorys of her will too, so maybe you and your gran could put something together between you and that would help her too as she recalls the happier times of your mums life and starts to fix those, rather than the sad ones in her mind.
I think her and your dad have done some grieveing, how could they not, but also they would have wanted to be "strong" for the children and just the practicalities of caring for you all, especially with such a young baby must have taken a lot of doing, which would have given them no choice but to just keep going, but grief is a strange thing and it cant really be put in a box, there are recognisable stages which happen at different times between people and not always the same stages to everyone or any of those stages to everyone
Your dads depression is an example of one, but it could also have been a mask for his anger, which is another common stage, - when we cant express it then we turn it in ourselves, we also find ways to justify feeling guilt, obviousely he wasnt guilty of anything, but that doesnt stop us finding a way to blame ourselves, so his depression could have the been the external face of all those parts of grief in which he got stuck.
So when people use the phrase they didnt "grieve properly" i think its important to understand that there isnt a wrong or right way to do this And while traditionally grief theory and grief therapy has concentrated on cutting the bond between us and the person who died, that doesnt work in reality, and then because it hasnt worked and society and therapists tell us we should be "cured" but we dont feel that we are. we start to imagine it is a fault in ourselves.
Something which might help you is a book by Klass, Silverman, and Nickman, called continuing bonds:new understandings of grief - you dont need to read the whole book there are some articles online that sum it up quite well. In brief it states.
"Grief isn’t about working through a linear process that ends with ‘acceptance’ or a ‘new life’, where you have moved on or compartmentalized your loved one’s memory.
but you slowly find ways to adjust and redefine your relationship with that person, allowing for a continued bond that will endure, in different ways and to varying degrees, throughout your life. This relationship is not unhealthy, nor does it mean you are not grieving in a normal way."
For me, reading it was a big weight lifted off me because i was some-one who thought not wanting to "let my husband go" or "move on" meant i was at fault and i would be stuck for-ever because holding on to him was more important than societys view, and in reality it actually helped me move forward because i could suddenly see that i didnt have to leave him behind but find new ways to take him with me.
For you, being so young and with fewer memorys this isnt so straight forward because it is difficult to continue the bond when you have so few memories to create those stronger links and those who could fill that gap are not able too because they were themsleves struggling with their own grief coupled with trying to continue to care for you and be strong for you, and thats not their fault at all they were doing what they thought was the best thing and always out of love, but for you the more long lasting memorys of her that are missing are keeping that void open.
The memorys you do have also involve her being there and suddenly gone - for your younger siblings she will always be some-one who died and was missing but they probably dont remember her as some-one who was ever there in the way you do, and at four its likely your twin may not have as many memorys as you do, as some of us do remember things that happened when we much younger than others do, - i have memorys of being 3 but my daughters memorys only go back to being around five.
So perhaps that is why you have always felt as though something is missing, more strongly or differently than they have.
So while memorys are a way for many of us to continue the bond with our loved ones there are other tools in the box - for some that is religion they think of the person looking down from another place or in many cultures honoured tribal elders whose wisdom can be conjured up or is known through stories passed down, but whether we have religion or not for others it is still through ritual - that could be visiting a headstone, or it could be something more personal, a place we visit that we associate with them,planting a tree and revisiting the tree to see how it has grown and how in spring it proves that life is always renewed in some way, we are all different but finding something which appeals too us either through our own cultural norms, adopting another cultures or creating our own but which is done in a way dedicated too them is still a way to keep the bond with them.I have a place we loved going and planned to retire too which i cant do, but i go back now every year and i bring bck a pebble off the beach - no-one seeing me knows what i am doing and no-one at home knows what the pebbles in a particular place in the garden are about, its not something anyone questions, so it doesnt have to be a big drama, just something you do as a memorial too your mum which exists only between you and her.
Another tool in the box is to live your life in honor of some-one - which means considering them in everything you do, your code of ethics for example or standing for a cause they believed in , supporting a charity they supported, etc - again as you didnt know her in that way, that is going to be more difficult for you but not impossible.
So definitely continue to write here, and also continue to talk to your gran so you can gather as much information as possible even if she is reluctant to create a memory box or help you do so, what she says is still a record which you can use to get to know your mum better even though she isnt here.
Perhaps you could write a letter too your mum - or a series of letters in a diary form, and also tell her about the things she has missed - your schooling, your college, your hobbies and interests - i know it sounds a little crazy but again no-one need ever see it, but i think it is something that could help, because had she been there you would have told her all that and so in a way you would be inviting her back into your life even without her being physically present and that could be a way to close the gap in it.