Thank you for patience.
Based on information given, there is approx £28,000 of capital gains tax due on sale of the property where you sister is living rent free.
Total gain (360,000-105,000) 255,000
Gain covered by private residence relief (129/231months) £142,403
Gain subject to capital gains tax (255,000-142,403) 112,597
Assuming Nan has other income that is covered by basic rate tax, the maximum CGT payable on this gain would be (112,597-12,000) 100,597 x 28% £28,167
Amount available after CGT (360,000-28,167) £331,833
Let property giving a gain of (400,000-3,500) say it is £400,000 for simplicity.
Maximum capital gains tax payable on sale of this property is
(400,000 x 28%) £112,000
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