Access could work very well for this. In the distant past, I helped many end users work with Access, and it sounds like a perfect fit for you for this type of need, especially since you already have some familiarity with it. There are templates that can create a database that can then be either changed to fit your need or referred to to determine how to do what you want.
At least to start, you'll want 3 tables. One that lists the conditions. It can have several columns, but based on what you stated above, start with: the condition name, the description, and a primary key. The primary key is just a number of each item as you add it, and is a better way to handle the relationship between the tables than the name or description. Access will try to add a primary key by default - let it. A second table will list the herbs, including a primary key. You can add more columns to these tables in the future, so no worries
Finally, a third table will list the the primary keys from the conditions table and the primary keys from the herbs table, as well as it's own primary key (3 columns - all numbers). This will list a row for each condition to herb relationship.
You can setup forms to make entering this data clean and easy. There will probably be a bit of a learning curve, but if you look at a template, it might help. As well as some basic queries (again, referring to a template).
I no longer have a license for Access (and don't know what version you have), and unfortunately, I don't think there's an easy and free way to post this to the cloud to share development. However, if your niece isn't familiar with what I've mentioned, let me know, and I think we can try and work something out in private messages or something, so I can get you to the point that you need. Once this basic setup is done, it will make maintaining your information easy, and will also make it easier to change the elements of the data in the future, should that be something you'd want to do. And I'm here to tell you -- if you don't get bored entering lots of data and give up the project all together, you will eventually decide you need to add to it's complexity! ;)
This may be more than you want to take on. You might want to wait see if anyone else already has a database with forms that you can use. If you're feeling adventurous, though, I'll do what I can to support you long distance
Full disclosure: I'm VERY rusty with Access, and it never was a primary development tool for me, but I should be able to convert technical information you encounter into common language to help you out. I'm a retired software engineer/data architect. Don't want you to think I can wave a wand from here, but I'd love to help you get to what you're trying to do if you decide to head down that road
If you have additional questions about the benefits/drawbacks of other alternatives, I'd be happy to provide you information you need to make an informed decision on what's right for you.
Best wishes! Give me a holler if you need anything.