1. At the outset, where the house is registered in your deceased grand mother's name, then it is still her house and falls to be dealt with in accordance with the rules of intestacy which apply where there is no will to be found. No entitlement to squatter's rights arises in the case of registered land as title is what determines ownership here, not the fact your father might have resided there for 14 years without paying rent. Secondly, under the rules of intestacy, where there are six children of the deceased, (I assume this is what you mean by direct descendents) and your grandmother was not survived by a spouse, then the house falls to be divided equally between the six children of the deceased, or whoever survived each of them. This means that your father only owns a one sixth share legally. This means that if the house is sold, he will only receive a one sixth share of the proceeds. Similarly, if he dies, his estate and his descendents will only receive a one sixth share of any proceeds upon sale, or upon the administration of the will of your deceased grandmother.
2. Accordingly, if you wish to move your father into something smaller, you should be aware that this house he is currently living in is not his. he potentially only has a one sixth share upon the administration of the will of your deceased grandmother.
It also seems that the house has not been registered - we have checked and it seems not. How can we prove who the house belongs to?