IMO, your husband needs to take charge and make a decision, both as to what is told and how care proceeds. He can fight the siblings later legally if they get cranky. MIL needs hospice care so she can remain happy for what time remains. IMO, she shouldn't be told of her cancer as her cognitive center is likely blown and she would only respond on an emotional level.
As a DIL, you should offer your opinion to your husband and then support him in whatever he does and refrain from engaging his siblings, directing them to him if they acost you. Remain positive and upbeat with MIL, no matter what happens. There might still be a few gems left there for both of you. If you haven't already, read some books on AD and being an effective care partner.
Regarding housing, if you have the means to pay for home care beyond Medicare's hospice coverage, that is one possiblity. Untreated, MIL will likely be gone within a year, likely less. My dad lasted around 4 months after the cancer got to his liver, even with chemo. If hubby decides on institutional care, with or without consulting with the siblings, he should investigate and line up a facility and then take mom there for "observation" or "treatment" or "therapy". Use an appropriate lie for MIL and siblings, whatever suits your needs.
Sometimes it's better to ask forgiveness rather than permission...
Good luck!
Pat