ALL of his children will be his beneficiaries - both your child and the children from his previous marriage - though I dont know whether you will be considered a beneficiary or not ... it depends on the state laws.
The exact split up will be determined by the trustee according to a lot of regulations ... I totally agree, you will need a lawyer.
Remember though - having a lawyer does not have to make it acrimonious - just that it is exceptionally complicated so will take a lawyer to handle it.
How are you coping emotionally? Losing a partner is so hard