Thanks for all the replies on this. It's interesting to read all the opinions. They differ so much and I agree with all of them tbh and this is why I am so conflicted! They are all valid points.
Few things to note, My Mum and sister love keeping secrets and lying or bending the truth frequently. I am a very open and honest person and this keeping her a secret for 12 years goes against my grain. My sister in particular lies about everything and I don't mean big things like this....it could be where she did her food shop or where she bought something or other small insignificant things. She seems to kept a thrill out of keeping secrets. I have no idea why. It baffles me. If you call her out on it she adds another lie or says she 'forgot' details etc
My Mum is controlling in a passive way. I believe she shouldn't have destroyed the letter as it wasn't her decision to make. With that said I absolutely don't want to hurt her because like someone said its obviously incredibly painful for her.
With regards to nieces and nephews, we both have children-not just my sister. I am also keeping this from them which doesn't sit well with me.
My half sister is 23. She has no siblings and her Grandad who she lived with (with her mum and Nan) died recently so I feel for her. I know this from her mum's SM account. Strangely I have not been able to find hers.
My Mum has made comments that they could be 'dangerous people'. I think she is just saying this to further put me off making contact. One idea I had was to hire a PI to get a background check first. As someone said re money yes they may be coming for inheritance even though apparently she has no claim as Dad left everything to Mum (this was checked with a Solicitor).
My Mum will shut down if I try to broach this with her and would see it as a betrayal. It is an awful situation to be in and I'm not expecting anyone on here to solve the problem as its so complex but thank you for all the replies and the respect you have shown me and everyone involved x