Savannah Chrisley and Jason Galanis seem very much in love, but his estranged brother says he isn't sure Jason is in it for the right reasons ... claiming his brother has always been a conman -- and he doesn't think he's changed.

TMZ's Charlie Neff sat down for an interview with Derek Galanis on Thursday ... and she asked him why he thinks his brother might have an angle when it comes to his new marriage.

Derek -- who wrote a book titled "Greed and Fear: The Galanis Crime Family" -- says everyone knows Jason is a conman -- citing his guilty plea to 7 charges connected with two different fraud schemes.

He says his brother and their father, John Galanis, have been conning people their whole lives ... and he has no reason to believe Jason's time in prison has changed him one bit.

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As we told you ... not only are Savannah and Jason married ... they're also expecting a baby soon -- and Charlie asks how Derek feels about the couple having a child.

Derek says he hopes the baby can convince Jason to change his ways ... because he certainly doesn't believe his bro has changed yet.

Derek also opened up about whether he wants to be included in Savannah and Jason's wedding ... and the details on Jason getting his sentence commuted.

You don't want to miss this interview ... head on over to our YouTube right now to watch it all.