With six in your party, there are a couple of considerations.
As far as lodging, you're obviously in a two-bedroom so there would be no additional room charge for an extra adult like you would have at non-
DVC resorts. But she will have to be listed as a guest if she's in the same villa.
If you get her a separate studio, she could get
DDP just for the days she's there, as Chuck said.
You didn't say how long your main party is staying, but buying DDP for your mother for the entire period may not be that big of a deal.
For example, if you're going to be there 7 days and she's just there 3, you would have 4 extra nights credits. You could use those for extra meals, or use them for a couple of 2 TS meals for yourself and DH. Or...use some of them for 2TS meals for her. There are lots of options.
If you are staying longer, obviously that's not as attractive an option.
The other thing you need to be aware of is the gratuity. With 6 or more in your party, there will be an automatic 18% gratuity added to all of your TS meals. And, as I presume you already know,
you are responsible for that tip -- it's no longer included in DDP.
ETA: That automatic 18% gratuity has nothing to do with DDP, incidentally. That will be added no matter how you're paying for the meal.