She’s over 70, so this makes you somewhere between…what? 30 and 50? Listening to the situation, I wouldn’t jump to the conclusion that she is being controlling just by this one incident, but that is neither here nor there just now.
If you had money put into your “safekeeping” by a relative and you lost it for any reason, your mother could be hearing about it from other relatives or by her experience she might fear this is coming. Families do this; I think the therapists call it “triangulation” and I think some call it “alliance building.” It’s when you have a problem with one person and you call around to the members of the common group you belong to (like the family or the social circle) and you pre-emptively get everyone riled up on behalf of your side of a dispute. I’m sure you know the drill; everyone has seen this at least in grade school if nowhere else. Your mother may also fear that her sister (or sister-in-law, whichever this aunt is) is going to come after her for the money.
My advice would be this: Have your mother tell you specifically what her concern is about your issue with her sister/sister-in-law. Is it that she is embarrassed? Is it that she is afraid her sister/sister-in-law is going to lose money she can ill-afford to lose? Is it because she is afraid that the sister/sister-in-law is going to hit her up for the loss? Does she even know? You’re going to have an easier time with your mom if (a) you take the trouble to find out what is bothering her before telling her to leave you alone about it and then (b) you speak directly to her concerns BEFORE telling her that in the future her harping will serve no positive purpose.
As for the money, yes, do not “try” to get a loan. Get one. That’s an integrity issue.
In the larger picture, I’d suggest talking to someone professional if you can find it for free or what you can afford. Whether you can or not, take a look at books by Melodie Beattie, such as “The New Co-dependency.” Libraries usually have them. Don’t be put off by these being books about dealing with addiction. They’re really not. They’re books about drawing healthy boundaries when dealing with other people, especially people in our families, in ways that keep the ownership of problems where they belong.
Co-dependency is a term used to describe a relationship in which loved ones (the co-dependents) try to take some ownership of someone else’s addiction (the addict is the person with the dependence on a substance of abuse). Taking ownership of problems that aren’t yours instead of giving only appropriate support to other people as they deal (or do not deal) with their own problems isn’t appropriate and it doesn’t work. Melody Beattie’s books are about how to draw boundaries so that each person minds their own business. I get the feeling they would have practical value to you as you turn your relationship with your mother more into a relationship between adults. It is going to take time and you may be the only “convert,” but in the long run it will help you have a more peaceful outlook on your relationship with your mom.