My six year old is telling lies

[Momo4321]

My six year old daughter ( adopted at 2y.) has taken to telling lies (Big ones) I know they have to go through this, but she is being non responsive to explanations as to why this is not right!! Almost a 'shoulder shrug' and a sorry.

How do I get through to her so that this is not the norm for years to come??
It's reduced me to tears!! Help please

What kind of lies? You say big ones... ones that would have severe consequences for other people or ones such as they went to mars the other day or their teacher is actually an alien?

Consequences for others.
Her last one had me and my 15 year old foster daughter shouting at each other. As I blamed her of opening a letter and then putting it in my bathroom bin!!
Now I've worked out it was her and asked her another 4 or 5 time to tell me the truth
She admitted it and said she just wanted to see what was in it!! She says she told the fib so she wouldn't get in trouble and when I reminded her someone else got blamed she shrugged her shoulders and said "yeah sorry" with no conviction!