Lisse,
I would proceed with your expenditure, any claim should be made strictly within 6 months the court has discretion to extend the period of 6 months within which claims can be made but the onus of persuading the court is upon the claimant, and it would be granted only in exceptional circumstances and on very rare occasions.
You say the relations have been aware of the death of your Father since March and it is now August and no action has been taken it is very unlikely it will be. It is probable the family members were not dependants of your Father and, as stated if they take legal advice they will soon be disillusioned.
A number of wills are challenged, but if your Father had mental capacity, the will is valid, and the family members are not dependants and there was no undue influence on what grounds are the family members going to convince the court to extend the claims period?