Makes sense if you think about it.
Everything Sagume says is wrong, therefore everyone thinks she's lying, therefore her descendants are liars too, because they all picked Mother's habits and speech patterns.

That's an interesting way to disown a child....

Seija: That's not true! That's impossible!

Some people still don't get it...

It's not literally everything she says that will be become reversed. Things that are already definite cannot become reversed, like she cannot say 1+1=2 and suddenly mathematicians declare that 1+1=4 from now on or say that this is a dog and that dog suddenly becomes a cat.

Her ability refers to things that have undetermined outcomes and possibilities, like if she says it's going to rain, it won't rain, or if she says this flower will not wilt, it will wilt.

Explanation on Touhou Wiki:
"Reversing a situation with her words

When she speaks about a particular situation or event, that situation will eventually proceed to reverse itself, through whatever means possible. If she speaks about an attempt to achieve a goal, it will fail in some way. If she speaks about something bad occurring, it will be resolved in some way. Due to this, she tends to not speak too much and always chooses her words carefully. Note that it is the situation or the event she talks about that will be reversed in some manner; it is not that the opposite of what she says must occur."

We knew it, Seija-chan.
post #2111945

game2009 said:

Some people still don't get it...

It's not literally everything she says that will be become reversed. Things that are already definite cannot become reversed, like she cannot say 1+1=2 and suddenly mathematicians declare that 1+1=4 from now on or say that this is a dog and that dog suddenly becomes a cat.

Her ability refers to things that have undetermined outcomes and possibilities, like if she says it's going to rain, it won't rain, or if she says this flower will not wilt, it will wilt.

Explanation on Touhou Wiki:
"Reversing a situation with her words

When she speaks about a particular situation or event, that situation will eventually proceed to reverse itself, through whatever means possible. If she speaks about an attempt to achieve a goal, it will fail in some way. If she speaks about something bad occurring, it will be resolved in some way. Due to this, she tends to not speak too much and always chooses her words carefully. Note that it is the situation or the event she talks about that will be reversed in some manner; it is not that the opposite of what she says must occur."

If we're talking about canon ability, sure, but if we're talking about them as an amanojaku that might not be too far fetched