I'm desperately sorry to hear this @canary1 - on the face of it, you do have the right to feel hurt and upset. As you say in one of your answers, you can't imagine doing the same to your kids, as you love them equally.
What I think you don't know (and this is where your parents have failed) is whether they are planning to gift something to you and hte other siblings at a later date. Maybe they (wrongly) think that you are financially sound, and thought you'd be happy to wait to inherit your share of the family home when they die? Maybe also, because that sibling has no children, your parents thought it unfair to leave him/her with nothing till they die, as she/he has no-one to pass it on to, whereas you can pass on any inheritance to your children. I know GPs who do this - they feel that they would like to leave everything to their GC but then if you have one child with no offspring, that's unfair. (It's a debate my siblings are currently having - should we ask our parents to gift some money to the one with no GC now, so that she can enjoy in her lifetime, but let all the other GC inherit the remainder of the GP's estate - there can be tax advantages in some places to skipping generations).
I'm not sure if you want advice, but I'd be tempted to write something along the following lines:
Dear Ma and Pa Canary, Sibling A has told me that you're planning to gift the empty house to Sibling D. I completely understand that it is your property and you are free to do with it as you want, and maybe there are things going on in Sibling D's life that we don't know about.
But hearing this news second hand has really hurt me very deeply. I have never expected to inherit anything from you, or for you to support me financially now that I am an adult, but when I heard that Sibling D is getting £XXX in value from you now, it makes me feel that you love them more than you love Siblings A, B and me. I'm sure that is not your intention, and I know you do love me, but it is very upsetting to see Sibling D getting such an amazing gift when we are working so hard to make ends meet financially.
I love you and all my sibligns a great deal and I will not fall out with them or you over this, but I think I have to let you know how much that this has upset me so that you can understand why I may need some emotional space."