As recipients of a gift, your brother and sister would receive the gift free of tax.
There is no tax on gifts in the UK.
You should be aware this gift would be regarded as a potentially exempt transfer for inheritance tax purposes and the seven years rule applies. In simple terms, if you survive for 7 years after giving the gift, the gift is exempt from inheritance tax and does not form part of the estate.
More information on 7-year rule can be found here
Some useful reading on gifts that might be tax-free can be found here
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