What are the grandparents rights?

My brother has a 2 year old son and the mom is keeping him from him because he has a new girlfriend! My mom is getting fed up because she never let's him come over for anything, we see him maybe once a moth for probably about 5 or 6 hours and he has to be home before 8pm they lived with us for a while but when they broke up she left, they have NO custody agreements, he pays child support but she won't let him see the baby because "he has a girlfriend".. My parents are fed up and want to know if THEY can get a lawyer and take her to court and get some kind of visitation? Please answer asap! My brother is by no means a sucky parent, he's safe here, everything! The only thing she has against him is he is dating someone new but it's been almost a year now, can she legally keep him from his dad just because he has a new girl?