I certainly think she needs some TLC.
Have you asked your boyfriend? As it's his family he should be able to give you more information about what exactly is going on and how his family would react to you wanting to help. I would be careful not to overstep boundaries as you are living in their house but if they would not be offended I would start small with something like grooming.
Tell them you're doing a spa day for your dog, whether it be at home or professionally done. If you're doing it at home simply buy a good quality shampoo and trim back the nails, if you trim a little bit off twice a week they should hopefully receed at a decent rate.
Start letting her out for potty breaks with your dog so she gets out more, offer to take her along for walks if they'll allow it. Even just little training sessions to help keep her mind active I'm sure she would enjoy. The food thing I would approach slowly, maybe highlight why you chose the food you're feeding your pup and see if they show any interest in switching her to a better quality. Even a cheaper alternative like Kirkland is healthier then a lot of the cheaper brands, or see if they'll switch to Purina One or the like which is still very cheap but better then Kibble and Bits
I'm sure she'd thrive under the attention and care and hopefully seeing how much happier and healthier the dog becomes will encourage them to step it up and work with her more.
Jasmine - Female Miniature Poodle - born Aug 15, 2010
Loker Delgado Von Stalworth - Male GSD - born Jan 26, 2012
Koda & Zazu - 5 year old male cats
Alex - Male Cocker Spaniel (rescue) - RIP Cuddlebug 2007-2010