Do you know can the declaration of trust be altered by my in laws or would I be able to transfer it when my youngest reaches 18,
equally split between our three children and my step son. I think this fair.
Effectively a new declaration of trust needs to be executed by those who are the legal owners (possibly parents and sister) and lodged with Land Registry; the new declaration would replace your husband's name with yours.
Parents' solicitors could do this.
Thus, you would then own a % of the property. I would at this stage perhaps do this.
At anytime thereafter you could transfer your interest to your children 25% each. This could be done by you simply executing a Declaration of Trust in their favour.
Under this approach you don't need to wait until youngest reaches age 18.
Tell their solicitor what you want to achieve and await their advice; you could always return to this website and someone will no doubt respond.