A couple of thoughts:
1) Amendment XIII US Constitution, Section 1:
Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.
It would seem that compelling one adult to pay for the education of another adult is “involuntary servitude”.
2) If the grandparents can afford to fund the lawsuit and support the 21 year old adult, why don’t they just subsidize the tuition?
Or the adult child could get loans or a job and pay for her own college.