Marriage is the easiest way, though I guess this is being asked because marriage isn't seen as an option yet.
The only way you could employ her directly is if your mother moved out to the Philippines for at least a year and employed her there first, but I guess this won't be an option.
You could try looking at a student visa, though your girlfriend would have to do 16 hours of day-time study per week during term time. The only issue is she should only work 20 hours per week during term time, though I guess it depends exactly what you mean by a full-time carer.