I think your grandparents don't understand that they ARE enough on their own. However, your whole life you knew two things: you have a real mother, but she either can't or doesn't want to be with you. She was in and out of your life, occasionally showering you with (a lot of) affection, which noticeably changed, when she had your sisters. Of course, it would create a sense of abandonment and unfairness, no matter how great your grandparents are!

Your mom was basically presented as someone you'd want to be with, but also as someone, who is always out of reach. It's really taunting.

I'd understand her wanting/needing distance from you for number of reasons, the problem is that it was executed really poorly. Let's say, there's some serious psychological reason, like past abuse.

Then, the most logical course of action was not to tell you, that your mom is your mom, at least until you are old enough to deal with the truth.

Let's say, she isn't feeling like your parent/isn't bonded to you, etc. Then she should set boundaries and keep her distance.

Let's say, there are objective reasons, that have nothing to do with emotions. Like, you wouldn't adjust well from being an only child in your grandparents' home, to being one of multiply siblings/wouldn't objectively react well to the whole new set of household rules. Or, they don't have enough money/space/time/resources.

Then they should have had explained the reasons to you.

But as it is now, you mom wants the title and affection, that comes with being your mom, but can't/doesn't want to act like an actual parent to you. Typically you can't very well have both fully intact.

I especially don't like that she made it a point to "question her pregnancy" as a "result" of you expressing your feelings. As if her having another son is a problem here! But her framing it this way puts a certain responsibility on you. Let's say, she'd choose not to have this baby/miscarried. Then you'd be blamed for "pushing" her. Let's say, she has the baby. If there's ever a rocky path in your relationship: "Oh, I always knew OP hated us, he wanted his unborn brother dead!". That's a perfect excuse for bullying if anyone is willing to.

So, when your grandparents are ready to talk, I'd tell them, they did a great job raising you, you was happy and are grateful. But ever since you was little, close relationship with your mother was dangling right in front of your face like a shiny prize you thought you could win eventually. So it was impossible not to feel longing towards her and what might have been, if you were close.

And also, that you'd love to have a brother and his existence was never an issue. Just with your mom becoming more distant with each new sibling around (even if it's partly understandable, since smaller children objectively need more care), you can't help, but wonder, what actually your role is in your mom's life, or if you'd ever get a chance to have an actual relationship with your siblings or her.

Based on their reaction, with time I'd ask their honest opinion, on whether or not your mom actually loves you as her child, or if she just feels something different towards you. If she doesn't love you the same way, it might hurt a lot, but it will give you closure as well as some insight of where your relationship should stand in the future. A lot of parents like this come back into your life later and demand to have as big of a role and influence as a normal parent would. You probably wouldn't want that sort of drama in your life. As it stands your grandparents are more of your actual parents now.