You ask this question and I don't know if it's emotional reluctance or monetary reluctance. Your niece is all raised by you, which means that she either has no parents, or her parents are completely unable to work, or you have no children, and when she is going to get married, you don't have to worry about it, and some things can be talked about.
01
If you have no children, your niece is your spiritual sustenance
As can be seen from the title, at least one of the families between you and your niece is flawed, so it leads to the formation of a de facto upbringing relationship between you and your niece, and in the case of you having no children, the niece is your spiritual sustenance for everything.
But now that she is getting married, you are afraid that she will leave you, will she not be emotionally attached to you in the future, will she not be old-age to you, after all, you have paid a lot to raise her. Since you say that you are a niece, then there is no legal adoptive relationship between you, legally you are not obliged to raise her up, and she is not obliged to raise you to grow old.
Therefore, you can seriously discuss with her, sign a bequest and maintenance agreement, agree that she will retire you, which part of your property belongs to her, and include the previous education expenses and the down payment of the house, so that even if she does not understand gratitude, you still have the right to request return in the future.
02
You can also make up an IOU for all previous expenses
My niece is getting married, and she will definitely have her own new family in the future, and the emotional relationship with you, no matter how intimate, will eventually fade, not to mention that you do not have a legal relationship to foster children.
Therefore, you can discuss the future problems, just say that you do not need her to rush to repay the money, but considering the problems of the new family, in order to protect her, I hope that she will write an IOU for her previous expenses before marriage, so that when she has problems in the new family, you can get back a copy that belongs to you, of course, the repayment time of the IOU can be as long as possible. If both husband and wife are filial to you after marriage, you can also directly waive the fee.
Of course, when talking, you have to talk from an emotional point of view, it is not easy to destroy the good relationship that has always been good, and perhaps this is not the result you want.
epilogue
People usually talk about feelings, but when the feelings become weaker, some nurturing graces may not necessarily take the initiative to repay, it is better to prepare in advance, protect themselves with reason and evidence, and at least protect their legitimate rights and interests legally when there is a problem.
In particular, the act of surrogate parenting without a clear legal relationship is more likely to go wrong, and can only rely on gratitude to support all processes, so preparing in advance is not a manifestation of unfeelings.