For CGT purposes, you are deemed to have acquired the house at its market value 4 months ago. If you transfer it back to either your sister or mother, you will be deemed to sell it at the market value at that date. Assuming you don't live in the house, any rise in value would produce a capital gain. You have an annual exemption of �8800, so provided the increase in value does not exceed that amount, there would be no tax payable by you (assuming you have no other capital gains in the year).
If your sister acquires the house, lets your mother live in it but does not live in it herself, any rise in value from when she acquires it until she sells it, would be liable to CGT.
If you give the house back to your mother, she would not be liable to any CGT.
The most sensible thing from a CGT angle would be for your mother to own the house as this produces no potential CGT. There may however well be other financial considerations to take into account.