Having a professional portrait made of Mom, or the entire family for that matter, is a great idea. It can't be both. For a Mother's Day portrait, the subject is obviously Mom, so she should be up front in the photo as you see it in your camera's viewfinder, so she will be prominently featured in the final picture. The next question is where to place your subject. Since May weather is mild, outdoors is a good choice, but don't put her in bright sun, since that will cause her to squint.
Open shade is a much better choice.
With an ISO 200 or 400 speed film you'll be able to get a good exposure and Mom will be more comfortable and look better than she would out in the bright sun. If you can make sure your camera's flash fires, set it to do so. Even though the camera may think there's enough light to take the picture without flash, putting some extra light on Mom will make sure she's shown clearly and also add a little warmth to her face. Try taking one frame with flash and one without. See which photo you prefer after you have the prints processed.