@CarinaBee
I haven't read all the very many replies, just your posts.
It isn't unreasonable at all that you feel the way you do, about any of it - your beloved sister, losing her, your DH, your SIL, your MIL, your SIL's choice of name...And you're right, therapy won't change your grief for your sister.
I do think, though, that it would be helpful to untangle your feelings and seperate the hurt you feel from different aspects of what's gone on with the ultimate aim of letting go of anger with your SIL and reaching a place of indifference regarding her. Not for her sake, but for yours: anger is a destructive emotion and ultimately the only person it's going to harm is you.
(Not saying you want to hurt anyone).
I think this is something you would benefit from working towards whatever happens with your relationship with your DH.
Your SIL may have chosen the name solely to hurt you, but chances are her reasons a bit more complex and I think you are giving her too much power to upset you.
You'll know better than anyone now that you can't change her behaviour and that no one is going to stick up for you, and I suspect you also feel a huge sense of betrayal and of being let down by your DH and even your MIL. With an absent mother and the dreadful loss of your sister at a young age, when she was also a mother figure to you, I think in addition to your grief there's the unhealed emotional wound of, possibly, abandonment? Of not being looked after, of not having someone in your corner to unconditionally look out for you, be there for you and stick up for you.
So, in addition to your grief about your sister and the need to protect her memory - the (understandable) place of sanctity she holds in your heart (and wanting/needing that to be recognised in your family unit), you're also holding on to hurts from various other corners of relationships with other people. Therapy could help you unpick these and once you're more consciously aware of them, reduce the power they have to hurt and destabilise you.
None of this is to say you bear any fault for your feelings, you don't. But ultimately, the only thing in your control is your own response to your SIL (and DH and MIL). I can imagine your SIL's actions feel not only a slight to you but to your sister also...
I wonder, what do you think your sister might say to you now if she could?
When people deliberately set out to hurt us, or when they simply don't care that their behaviour does, every action feels like a fresh attack on a wound that isn't given the chance to heal. There only thing we can do then, is to put up a shield around that wound and the way to do this is to learn how to emotionally detach from them and their actions.
Your SIL's actions don't affect your sister and they won't alter her importance in your life. The only way her actions will hurt you is if you let them. It's difficult, I know, and detaching yourself from her won't happen overnight but you can do it, you can take her power over you away from her. She will feed on the family drama created by your pain and betrayal so...take it away. Adopt a policy of indifference and work to reach a point of indifference within yourself.
She is not your family, your DH is, your children and sisters children are.
Re your DH and MIL - the people who love us let us down at times, they can't always help it. It may help you to try to understand why your DH and MIL feel unable to stand up to your SIL and the deep rooted emotional and family dynamics at play there.
Or perhaps your DH is just a dick.
Whatever. Your choice is this: let your SIL have power over you and your feelings, or don't.
Separate your sisters memory from your poisonous SIL and try to identify the different emotions in the surge of pain you're feeling. Once you've done, you can start to lessen it's power over you, too.