Can a sibling give his estate share to another sibling

Sure he can. There may be tax consequences depending upon what is being gifted to the sibling, but he can do it if he wants to.

In some cases when a person does not want an inheritance they can 'disclaim' their share - I doubt that under the documents a disclaimer would give all to just one sibling (you'd have to read the document). However if the person receives their inheritance they can give whatever they wish to any other person - it is a gift. It will likley effect the giver of the gift in that they may have to file a gift tax return and perhaps pay gift tax but only if they themselves have a rather large estate.

Yes. The sibling can assign their interest to another sibling or alternatively file a timely disclaimer. The disclaimer treats the sibling as though they never existed. Thus the estate would be split by the remaining siblings equally.

It depends. Some Trusts do not allow assignment of beneficial interest by beneficiaries. Absent such a preclusion, and absent the sibling assigning the interest having creditor issues (viz., a fraudulent conveyance) it is possible to gift the interest to anyone, including a sibling. If the share is worth more than $10,000.00 a lawyer should be consulted as the gift may require filing a 709 (gift tax return). A CPA may also assist in filing the 709, as many CPA's file 709s for their clients.

If he gives it to one of the siblings it is going to be subject to gift taxes, unless it the value of his share is less than $14,000, which the annual gift tax exclusion. As a practical matter what really happens is that when the sibling who made the gift of his or her share to the other sibling dies, it reduces the available estate tax exclusion which for 2015 is $5,430,000. For most people it is not an issue because their estates are worth less than the exclusion. As an alternative the sibling who would otherwise get the interest in the estate can disclaim his share by providing a disclaimer in writing within 9 months of the proposed transfer provided he has not accepted the property or any of its benefits.

If the sibling doesn't want his share because it will disqualify him for government benefits a first party special needs trust may be a better way to go.