I'm very late in replying, but wanted to pipe in as a former hospice nurse. It might be worth checking another hospice, if there is one in your area. Determining hospice eligibility is the job of the hospice medical director (a physician), and is not a black-and-white affair -- there are no rules that say a hospice cannot take a patient with ALS who is on bipap. There are eligibility guidelines (based on diagnosis), but even these are not set in stone. In the end, the hospice doctor has to certify that, in his or her opinion, the patient has a six month or less prognosis "if the illness runs its normal course" -- and to support this opinion with clinical fact. So, for example, a patient with ALS who has been vented for some time and has a g-tube and has been tolerating both interventions well and "maintaining" over time would probably not be eligible -- but if a person with ALS, regardless of the bipap, is showing signs of clinical decline (ie, losing weight, losing motor function, developing skin breakdown, developing respiratory infections, etc.) that would support a pretty limited prognosis, he or she might very well be eligible. It is entirely possible for two hospice medical directors to disagree on eligibility (even within the same hospice). Thus, possibly worth requesting a consult from another hospice.
The other piece is just finances. If a hospice takes on a patient with ALS, they are paid a daily rate (in the $100-200 range depending on geographic location) by Medicare, and are required to pay for the medications and therapies the patient needs. This would include bipap...so if the bipap is expensive, some hospices will not want to take that patient. But there can be ways around this: you could pay out of pocket for the bipap, or if your insurance is other than Medicare, sometimes the insurer will agree to keep covering the bipap expense separately from hospice (this is called a "carve out").
To me, the main upside of hospice was the on-call component. Assuming patients want to avoid going to the hospital, hospice is one of the few services that will actually come to the house and help you get through some sort of crisis (like a pain spike, or a pneumonia, or whatever) without going to the hospital. Even in the middle of the night! And there is a hospice "level of care" called "continuous care," where they will even provide 24/7 care, for a short period of time (like a few days), until the symptom is brought under control to the extent that the family system can manage it.
Our hospice has also helped with patients who wanted to be withdrawn from a ventilator at home. So if removing the bipap is consistent with your sister's wishes, they could provide the expertise to do that while keeping her comfortable. But the bipap should not be an absolute barrier to hospice.
Best of luck with all you are facing!