If it was sold for a gain, it doesn't matter what you call it, investment property or a 2nd home. Either way, you have a long term capital gain. Each sibling will report his/her share of the gain on his tax return. You cost basis is the Fair market value (FMV) in 2010.
If it was sold at a loss, a capital loss on investment property is deductible. A capital loss on personal use property (e.g. 2nd home) is not. So it depends on how the property was used for the last 11 years. If it sat vacant, it is considered investment property.
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