One property, one sale.
Also note that, if you are a US person subject to US tax law, you calculate the capital gain according to US tax law, even if the gains calculation in the property's location is different.
You need to calculate your cost basis. If you purchased the property with your parents originally, and you paid 40% of the cost, then your basis starts there. If the purchase price is $500,000 and you paid $200,000, then your cost basis starts out at $200,000.
You inherited 30% of the property when your parent died. Under US law, you receive a stepped-up basis equal to the fair market value on the date they died. You give that value as $2.5M, so the 30% you inherited has a basis of $750,000. So your current total basis is $950,000.
(However, you can also claim a basis adjustment for any permanent improvements to the property, such as remodeling a kitchen or bathroom. If the work happened when you were a 40% owner, you can add 40% of the cost to that part of your basis. The stepped up basis on the inherited part doesn't change.)
What is the selling price? Suppose it is also $2.5 million. Your 70% share is $1.75 million, so your taxable capital gain is $800,000. If you lived in the home as your main home for at least 731 days of the past 5 years (does not have to be consecutive), then you can exclude $250,000 of gain from tax, or $500,000 if married filing jointly, and then you pay capital gains tax on the remaining gain.
The point about a loss is not really relevant since you are likely not selling at a loss. Since your basis on 70% is $950,000, you would only have a loss if the property sold for less than $1.35 million. If that unfortunately did happen, you can't claim a deductible capital loss since this was personal property.
You need to report and pay the capital gains tax on your US tax return. If you also paid capital gains tax where the property is located, you can claim a deduction or credit on your US tax return for the foreign taxes that were paid on the same income.
