This is a very complex area and you should discuss it with the lawyer you hire to transfer the property.
1. Selling it to you for 65K ie under market value. One of your sibs is bound to suggest that. All sort of things get deemed in that case and I do not advise it. Like if they go into care it is still deemed to be theirs.
2. sell at market value - are you really planning to give your parents 99k ? Then it is straightforward. IHT irrelevant as it has been disposed of, and anyway is still under the IHT lmit of �300 000
however - capital gains - it is a disposal for CGT purposes however the CGT payable on disposal of your only residence is zero.
That is your parents CGGT position - nothing
however after a few years, you three sibs will have a CGT liability as the value of the house goes up. This will kick in if and when you sell it. however if your parents die IN the house and then yousell it, there is a little known exemption - dependent relatives die in a house owned by A B and C, then A B and C do NOT pay CGT under one of the finance acts. Obviously you will have to check that this exemption still appplies. Rent should NOT be paid.
If they go into care, then clearly they dont die in a house owned by relatives and the exemption doesnt apply and you three get dinged for CGT. May be worth it.
This is a complex tax area - not surprisingly - as the govt has worked hard to prevent parents giving the house to the kids and then going into care -which is very expensive.