I never adopted my step-daughters even though I raised them
Can I adopt them now even though they are adults as I fear a
Legal problem with my family if I should die and I want my 2
Girls to have what I have got

They don't have to be adopted to inherit from you - in some States, yes, if they're adults you can adopt them without the consent of the natural parent(s).

Handle it by Will - you don't have to be the legal father to make bequests.

For that matter, you could leave your estate to your dog - doesn't have to be blood family.

They are adults. Write a will. Or a living trust. You can leave your inheritance to whomever you wish. If there is no will or living trust it will go to your kin.

In the US each state has its own laws about being able to adopt a adult, you will have to see an attorney in your area But as noted all you need is a will.

Ur step daughters are very lucky to have you.Just shared. Consult a lawyer BTW.