Friends, don't let someone else's undealt with issues begin to make you feel like you have issues. This kind of person wants a reaction and thrives in dysfunction -- not necessarily because they are a bad person, because this is what they know. This is how they lived, and if they have never gone back and allowed someone or God to help them see it wasn't healthy, they'll stay in it, and pull the people close to them into it.
Sometimes we have family members that do this and we have gotten healthy, but they have not. And often, they have a hard time with that. Well, in comes the boundary, and you can trust that God will help you stand your ground as you make it in love, and just speak the truth. Be prepared though, because often when that person sees their tactics aren't working anymore, it can get bad. It can get ugly. But you have to remember, it's not you, it's just God bringing to light some of the wounds, hurt, and ugliness in their heart that needs dealt with.
Recognizing the toxic behavior ahead of time will save you years of heartache. As I said, if we are used to toxic behavior in our relationships, we will almost crave it, to feel normal. But all it takes is looking back over your life at the relationships you’ve had, and how your family operated (or didn't operate at all), and if you know it wasn't the greatest, or you know there were some issues, bringing them to God and asking Him to show you what needs to be dealt with, so you don't carry those things or behaviors into any more relationships with you. A Christian counselor is a game changer. Their job is to literally look over your whole life with you, and help you receive healing in every area, for what's happened to you, what's been done to you, and what you might have done to others.
But if you are the one dealing with someone toxic, don't take it. Sometimes the best thing you can do is put a boundary in the sand that will push them directly into the healing arms of Jesus. Even if it hurts or is hard to do, it is the GOOD, right, and loving thing to do. For you, AND them.