Is Matthew 12:43-45 a hint that we can have spirits enter in us and cause us to feel, think, believe or even do certain things that go against God? It's not the same as possession is it? Also, Are the people who believe in reincarnation, and feel as if they remember having past lives, under the influence of some of these unclean spirits that may have inhabited the people in older generations of their bloodline? Does this passage in Matthew serve as part of an explanation as to why they could have felt like they have lived other lives? What other scriptural proof is there for explaining what's really going with the people who sincerely believe they've remembered having past lives?
People believe in reincarnation because they are not interested in truth. "The coming of the lawless one is according to the working of Satan, with all power, signs, and lying wonders, and with all unrighteous deception among those who perish, because they did not receive the love of the truth, that they might be saved. And for this reason God will send them strong delusion, that they should believe the lie, that they all may be condemned who did not believe the truth but had pleasure in unrighteousness" (II Thessalonians 2:9-12). People don't want to accept responsibility for their choices, and for some, blaming them on imaginary "past lives" is preferable.
During the time of Jesus, demons did possess people, but that ended in the same era. See "What About Demon Possession Today?" The illustration Jesus gave in Matthew 12:43-45 was to teach us what happens when a person tries to only remove sin from his life without replacing it with righteousness. Jesus states that is how the Jews were trying to be "righteous" before God and why it was not succeeding.