If so, I would say it is neglect under the laws in most states.  The D15 is a minor and should have a LEGAL adult oversee her for absences of this length.  Her brother (at 18) may not be in a position to make legal decisions on her behalf.  For example, if SD is in an accident or has an urgent health issue, I am not sure that brother can make decisions on her care since he is not the legal guardian.  

Not to mention that a  lonely 15 year old girl can be very vulnerable.  Whether it's common criminals/thieves who come to the door, to on-line predators - she's at risk. 

You are right to be concerned about this girl's physical and emotional well-being, therefore it is your duty to report it to the appropriate authorities.  In most communities there are ways to do this anonymously. 

My experience with CPS is that unless there is a lack of food or other necessary things, they don’t see it as neglect. If you can make a case that her mental health needs are being neglected, that’s your best bet.   BM here left 14 yo SS with a 17 yo for two weeks and left the country and neither court nor CPS had an issue with it. 

what did police say when you called them?

I also don't think it would be considered abuse/neglect given that she is 15 and technically with an 18yr old (even if they aren't home much). I was actually in this exact situation when I was 15. From age 10 on, my dad left us alone for weeks at a time. When I was 11 and my sister was 13, we were sent to live with my aunt, and then put into foster care. However, after 2 years, when my dad had "proved" he would be around more, I was allowed to live with him again. He did kind of stick around more for about a year, but by the time I was 14, he would leave us alone for weeks at a time again. Nothing was done, and this continued until I graduated high school. I essentially had no adult supervision my entire 4 years of high school, since my sister was a troublemaker and bad influence. It's a miracle I turned out as a normal, functioning adult.

Does this girl have another parent she could be with instead? If she does, that changes things. (I did not).

What would you like to see happen? Because this ​​​​​​is terrible parenting, and it is adversely impacting this teen girl. She's not doing well in school, isn't enjoying extracurriculare and friends, and is having issues with her mental health. That said, your SD is 15, not 11 or 14 and does have an adult relative present in the home. So while the situation is nebulous, the outcome is likely to be poor.

You could seek full custody,  first gathering documentation of dates BM is absent, grades, etc and then filing for emergency custody while BM is out of town/state/country. Or,

You could take the middle road by having your DH 's attorney draft a letter to BM outlining her absenteeism, his concern for the impact it's having on the joint child, and requesting a plan for mental health treatment for SD. This might scare BM straight. Or, 

You could say nothing, keeping in mind the impact a semi feral teen girl used to having no rules would have on your home if she came to live with you.

I was one of those SMs who got stuck raising someone else's hardbroke, neglected kid when YSD14 came to live with us. Her parasitic mentally ill mother would leave her daughter alone for weeks in order to visit her sugar daddy a few states away. SD would run wild, skip school, etc. When DH found out, it was laughably easy for him to get full custody BUT, trying to civilize, educate, and get mental health help for YSD was  an exhausting expensive slog that ultimately helped very little. If I could go back, I would never have encouraged my DH to "do the right thing" . Having a big heart can be a huge liability for stepparents, so think very carefully before moving forward.

It should be, but is it? Probably not. Having the 18 yr old there, as long as he is home at some point every night, covers BM. Check your states rules though. Some states don't have any restrictions on things like this and charges will only be brought IF something bad happens. 

My state is one of those. There is no minimum for children to stay home alone, however if a 5/6 yr old is left alone and get hurt, child endangerment charges will be brought upon the parents. 

Nope, its not abuse according to the SpermLand CPS.  My SIL was in exactly this situation when she was 15.  MIL and IL moved 4 hours away and rarely saw her.  There was no food in the house and the house was a shithole.  Not that it was any differenc when my ILs were home.

My DW called CPS and they told her "Your sister is 15yo, can find food and can clean the house if it is dirty."  So, I doubt there is much that will happen even if someone were to call CPS.  Though I would be calling daily just to be a PITA and to paint BM with the appropriate shit colored paint.

Good luck.