Snarf, so sorry to hear about your mum, it is awful losing a parent, but especially awful to lose them both. 
I lost my dad just after DS1 was born and my mum shortly after DS2, both in my thirties. I went through a marriage break up and all the trials of having 3 DCs, meeting a new DP and all the tricky stuff that entails, with only really Mumsnet for company.
I made some wonderful friends on here who I have since met up with in real life and we have a secret group on Facebook where we support each other. I really think people who are in the same boat are the best people to turn to when times are rough. While your siblings are experiencing it all with you, they need your support and aren't necessarily able to give it, whereas on here, everybody sort of chips in and supports each other.
I have 2 siblings and no wider family, and I have a handful of friends, but again, they're more the 'coffee & cake' type than anyone I can really lean on in a crisis. You might find that your younger siblings can support you too.
Friends who haven't experienced a loss like this can't really comprehend what you're going through, so even those you do have don't really seem to 'get it'.
I honestly think MN is the best friend you could need at times like this - people who have experience of the same thing, who are caring enough to support you without trying to 'fix' anything if it can't be fixed and who are there 24/7.
As Lulu said, it's never going to be the same, but you will cope. I try to think that they are still with me, because I am 50% each of them, so when I need my mum I take care of myself as I know she would have done, get snuggled up with tea and a blanket and tell myself it will be ok.
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and a warm snuggly blanket for you x