Have you considered each sibling buying a portion of the new property comensurate with their contribution to it, each can then decide how much they want to contribute. Each would then raise the money themselves, whichever way they want, savings, additional finance on their own home. Then the siblings financial dealings are independant and no one need know the others incomes life insurance situation etc. An agreement would be drawn up between all persons having a stake in the property regulating the arrangement. Each sibling is making an investment which can be recouped when your mother no longer needs the property.
If you get a mortgage on the property you mother is considering purchasing then everyones financial information has to go on the form and your mother has to go on the mortgage, you all go on the deeds. Everyone will be jointly and severaly liable for the whole amount of the mortgage, could you cope with repayments if all of your siblings stopped paying, would you want to?
I think you are suggesting that after you buy the house that your mother goes for equity release, these schemes are a very bad idea and quickly erode the value of the property. There was a set of personal experiences in last saturdays guardian newspaper if you can get a hold of it. Consider your options very carefully and get some very thorough legal advice.