"I'm not sure what's true love" vs. "I'm not sure what true love is"

Are all these sentences grammatically correct? Which ones are more common?

"I'm not sure what's true love" vs. "I'm not sure what true love is"

"I don't know who's your brother" vs. "I don't know who your brother is"

"The problem is who can we get to replace her" vs. "The problem is who we can get to replace her"

I know as a general rule, we shouldn't use question word order in indirect sentences (e.g. "*He didn't know where was I going" is obviously wrong.) But there is also this example that is definitely correct in this way: "I'm not sure who is in charge." while I think it would be wrong to say: "I'm not sure who in charge is."

I don't know how to explain this. I also have no idea about the first three sentences!

Solution 1:

1 - What is true love? - Here we have an independent question. Independent questions have either inversion or the question auxiliary to do.

2 - I don't know what true love is. - This is a subordinated question and they have normal word order, subject, verb. No inversion or question auxiliary to do.