Worried for my mom..............?

So this is going to be quite long but i have to ask! My mom is married within 2 brothers and her father-in-law & mother-in-law seems to favour his other daughter-in-law more than he does for my mom. They will also buy things for her son but never for my moms daughters (me and my sisters). My mom seems too always get stressed over this because she takes so much care of my grandparents while the one they favour doesn't give a ****. Seeing her depressed makes me and my sisters feel bad too. My father also never understands her pain, only my sisters and I understand her. No she cant divorce him because her parents are not alive and she doesnt want to be a burden on her brother and sisters and she doesnt want too because she loves us too much. But overall we are all always happy together but just occasionally some problem like this will rise.What can she doo?