You have exactly the same standing as any biological grandchild. The original purpose of legal adoption was to secure and legitimise the relationship between child and adoptive parent or parents so you are as entitled to an inheritance as much as the other grandchildren.
Your uncle is horrible, frankly. Greedy. I wonder what the other relatives think of him making such hints. I've not come across such an attitude and I've known families with adopted children, I myself was adopted. Whatever ups and downs I had with my family on either side when young I was always considered niece, cousin or whatever, never 'adopted niece'. They were all delighted for my long childless parents when I came along as a tiny baby.
I had one childless aunt on my dad's side who left me some money in her will, along with her one biological niece, there was no problem about it. My cousin and I are friends to this day, aged 75 (me) and 83 (her).
I also inherited a share of my biological mother's estate :-). That was a surprise! She was a widow and had had no children (by choice), with her husband. All she left, apart from a charity bequest, was divided equally between her two nieces, two nephews - and me. They didn't even know I existed! If she hadn't left a will I would not have been entitled to anything at all, probably wouldn't even have known she'd died. It was touching and very nice.
Hold your head up, let the law take it's course; it won't happen overnight, takes time to process probate and sell a property, etc, but you will get your share.
If I was a violent person I would happily punch your 'uncle', who is not worthy of the title, but as I am not, I won't.
Good luck to you and I hope you enjoy the inheritance, however much or little it is. Your granddad would have wanted you to, so would your parents.