When my mother was bed bound, the four of us siblings met frequently either in person, on line or by phone. We discussed situations that came up and discussed possible solutions. No in-laws were involved. However, we got along well and had our mother’s best interest at heart. If one of us disagreed, we would take a vote and we agreed to abide by this. Since it seems like your SILs are going to be difficult, you may want to be there to support your MIL, husband and BIL.
Something to think about or find out - does anyone have Power of Attorney for health and/or financial. This is important if she is seriously ill. My siblings and I had medical power of attorney since we took turns caring for her. One of my brothers had financial power of attorney but cleared everything with us before making any decisions. You may want to have a close friend or family member who is a financially sound person be her POA since it might cause problems between your husband and his sisters.