Usually this type of bequest is not a probate issue. What I mean by that is a probate is not left open until a grandchild who may be unform receives money at 18 or 25, etc. Instead a trust is set up to handle the investments and ultimate transfer. The elements of the trust should not negate the will (such as "Grand-daughter gets $10K when she turns 18) but one needs to look to the trust document for an answer to this question.
Basically he can't withhold the money. As a beneficiary of a trust she can enforce the terms of the trust against the trustee. Just be sure you know what the terms fo the trust are before pressing the issue even if the trust is just set up by the will (testamentary trust).
First, your sister should obtain a copy of the will or trust. Second, if the document states, as you believe, that your sister is to inherit $10,000 on her 18th birthday and her 18th birthday has passed, your sister should send your uncle a certified letter demanding he release the $10,000 to her since she is now 18 and the will or trust states she is to receive the money when she turns 18. If your uncle refuses after having a reasonable time to turn over the money (e.g. 30 days), then your sister could file a petition in probate court to force him to hand it over. For this last step your sister should consult with a qualified probate attorney.
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