1. This is correct as 'My sister told me her secret' or 'My sister told me what her secret was' 2. You should replace the first 'my' with A 'A friend of mine told me where I can buy tickets.' 3. This is correct. But think about the use of 'will', it might be better with a different modal verb such as 'can'.

1. This is correct as 'My sister told me her secret' or 'My sister told me what her secret was' 2. You should replace the first 'my' with A 'A friend of mine told me where I can buy tickets.' 3. This is correct. But think about the use of 'will', it might be better with a different modal verb such as 'can'.