My mother, was cremated on her own wishes without flowers and service, two of the siblings now want to bury my mothers ashes under a TREE! My mother did not want to be buried she made this clear to all four of us unfortunetly the two siblings who want to bury her have the ashes is their anyway we can stop them burrying my mums ashes because that is far from what she wanted and its upsetting the rest of the family what they are doing.

Did she have a will and state this in the will?
Who is executor of her estate?
If she had a will, did she make it clear that wanting cremation also meant not burying the ashes? Many people don't want burial, but don't care where their ashes go.
Who paid for the cremation? Often the ashes go to whoever paid for them, unless the will or the executor(s) want otherwise.
With both my parents, we siblings divided the ashes equally and each did what we wanted with them, since neither parent specified.

Being buried in a cemetery plot and having her ashes buried are different things. But you will need to file a law suit to stop them. That means consulting with a solicitor. I'm not sure we have any solicitors as members here. But all any could tell you is whether you have a chance of stopping them, and you do. But to start the process you need a solicitor.

Unforuntetly they already done it now, they have planted her under a tree, have made a fake will etc, anything we could do?

Again, this would require a law suit. If the ashes were buried in an urn, you could file suit to have them dug up. But if the ashes were poured into a hole in the ground, then I would just give it up.