Am I right in thinking that if there is a validly executed will, made when a person was of sound mind (verifable by medical professional), then a sibling would have no real grounds to contest a will?
Do siblings have an automatic obligations under law to provide for each other? I know that parents have duty of care obligations towards their kids, but do siblings have that too? I wasn't aware that they did.
No siblings do not have any such obligation. And parents can decide to disinherit their children. There is no legal obligation to leave something to one's children. But if a parent is 'unfair' to a child that child may have a 'chance' of something by contesting the will.
Let's be clear about this. I am sure that the answers provided above are legally correct.
But the fact that there are no grounds will not stop someone from tying up the whole estate for years in a groundless challenge. They may well get legal advice that they have no grounds for such a challenge, but that might not stop them making that challenge.
The testator should take steps to minimise the challenge such as
Get their own doctor to certify their mental competence
Use their own solicitor to make the will
Visit the solicitor's office on their own i.e. not with one of the beneficiaries
Maybe even make it clear while they are alive what their intentions are.