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If your Mother were to pass on first, you are correct in that under the current arrangements, the whole of the property would pass to your Father and it could then all get swallowed up in the payment of Nursing home fees.
There is, however, a way of securing at least 50% of the property for your inheritance. Your Mother and Father can "sever the joint tenancy" in the property, meaning that they both own a separate 50% share each in the property. They can then update their Wills, leaving their respective 50% to you and your siblings, with the proviso that your Mother be entitled to remain living in the property for as long as she wishes, if your Father were to pass on first.
Hence, doing it this way, only 50% of the property will be swallowed up in Nursing home fees.
For the above to work, your Father will need to be mentally capable of signing a new Will, which I hope he is. If he is not, then your Mother can go ahead and sever the joint tenancy on her own, and can make a Will leaving her 50% to you and your siblings.
I hope this assists and sets out the legal position to you.