This is an interesting question, as the future of these two characters is never specifically disclosed to us. What we do know is that in the penultimate segment of the novel, Max and Liesel are reunited, and when they see each other again, we are told that: "They hugged and cried and fell to the floor." It is the final segment of the novel that tells us about how when Death went to take Liesel away, she was in Sydney and she is an "old woman." This therefore very strongly implies that she married Max and moved to Sydney, free to enjoy a happy and long life before her final death. However, the book, as with so many other issues, never spells out her destiny precisely, and we are left uncertain of whether Max and Liesel actually married or not. All we can say is that the narrative strongly suggests this outcome.
There is no way that they would ever get married. Liesel fell in love with Rudy, not Max. I think that when the author writes about the two of them, they are referred to more as having more of a sibling relationship rather than romantic one.
The book never says Max and Liesel got married. You would expect Death to mention this, but instead he leaves us pondering about it. You can assume they did or assume they didn't, but all-in-all the book does not say. If you are worried that you may be tested on this, don't worry, because you probably won't. However, if you are you can just say that the book does not answer this question. By the way, I know the study guide for this book on eNotes says they got married, but this statement was just assumed. Think what you want, but all we can say is that the book does tell us whether they did or did not marry.
Even from the beginning Leisel seemed way too young for max. Although it is a possibility.