I think you should think if it in whatever way matches your view on how and when you yourself want to look at this. It also depends on what kind of physical means (paper, apps etc) you have chosen to use. What I see is two main avenues (and of course there are combinations and variations):
1) If you are using a system with separate lists (paper, simple apps), all you have in concrete terms is these lists, and you review each one more or less often. You have lists of actions, lists of projects, lists of areas of responsibility (aka areas of focus), lists of goals, lists of visions etc. If the is the approach you have taken then your question really boils down to "Which of these lists" should I put this thing on?" and "Does it matter which one?". You will probably review some of the lists more often than others - so how often do you want to review it? Another consequence of putting things on the same list is that the combinations of items on the list may either confound your overview or enhance it - so in what "context" (in the general sense) do you want to review this? It becomes a matter of making it possible for you to conduct insightful and creative reviews.
2) If you are using a system with hierarchical capabilities (such as an unlimited-level outliner app or a multi-level power app) you will have noticed that some - but not all - of the above things lend themselves extremely well for hierarchical representation, whereas others do not. For example, you may have actions that fit into a project that fits into an AoR that fits into a group of AoRs or a medium-to long term goal. Then by using that in a tidy way you can build reviewability and constant awareness into the very fabric of the hierarchy - and you can review the higher-level items separately if you like. But other kinds of high-level goals (and even some types of areas of focus, depending on how you define them) are "all-pervasive". They do not fit into a hierarchy because they can affect more or less anything across the whole hierarchy. Those all-pervasive goals you need to continue using separate checklists for (or, conceivably, tags of some sort), but let us leave those out of the discussion for now. If you have taken a predominantly hierarchical approach, then your question boils down to something similar as in the first case "On what level should I place this item?". This time frequency of review is not an equally important deciding factor, since you see these higher levels all the time and during all your normal weekly reviews. You probably even do these reviews one such block at a time. But just as when you look at a higher-level checklist (in the previous case), you still need to decide what things you want to see together on the same hierarchical level (e.g. when you look at a list of collapsed trees)? Does the item fit in with the other things at this level? Do you want to look at these things together? Do they all provide a good overview or are they ill-matching, confusing?
I myself am using a hierarchical approach. For smaller projects I use "tasks with subtasks" in order to keep my project list tidy. At the project level I keep the more significant projects (all marked by AoR) and also some "projects" that are in fact permanent AoR containers for single actions. I have ten areas of responsibility, all of which are "hierarchically" defined (i.e. nothing can belong to two AoRs). This gives me a stable, graspable view at that level. One level higher up I have reduced this into just a handful of containers, three of which represent "groups of areas of responsibility" (business, non-profit, private - all on-going stuff) and some of which are major long-term concrete new goals (completable "super-projects" that I will check off one day after several years or so). For my "all-pervasive" type goals (e.g. being happy etc) I do not have a formalized approach (just some checklists and notes for some aspects, but that's about it.)
If I were in your shoes I would treat "Mum" as an Area of Responsibility, under which I would put various projects and tasks. (That's what I did while my parents were still alive.)
