Oh love I wish I could reach out and give you a hug.
I am so very sorry for your loss and what you're going through and agree with previous posts.
Counselling/therapy - it can just help you process your grief and your feelings. Grief over our loved ones never leaves us, we learn to live with it, our lives grow around it and in doing so lesson the pain. Eventually, we find some peace and acceptance, which I think is what you need.
As others have said, you're also grieving what you missed out on. Your guilt is normal - there's a level of reconciliation always with grief.
Reconciling our sorrow and pain and love with anger, guilt and feelings of loyalty. Sometimes, just having someone present we can pour it all out to and who gets it can be healing on its own. Other times, a counsellor helps work out the conflicting feelings, deal with any anger (which can emerge at odd times) etc.
In your case, it sounds like a therapist would help you recognise your grief (and relationship with your mum) as every bit as valid and important as your younger siblings. They would also help you deal with those feelings where you worry acknowledging them means ascribing blame to your mum and any regret or anger you feel.
In helping you find the emotional space to understand and deal with all this, they'll free up blocks which stop your grief 'moving on' (read: evolving). All this is done is a safe place where you can cry or rage or question as much as you need to.
Finally, counselling doesn't have to be done and dusted in one set of sessions.
Sometimes, we need to take a break from counselling for weeks, months or even years, and come back to it later.
A very wise person told me once that grief is like waves crashing on the shore. It keeps comes coming - and often in waves, often surprising us- but overtime the power and strength of the waves get less.
At the moment you're completely at sea, but you'll find an anchor and a counsellor might well be that anchor. X