Serious bussiness help.?

My wife just passed away, leaving me alone with my infant daughter and 12 y/o stepdaughter. My brother-n-law has asked for me to grant conservatorship of my step-daughter because she hasn't known me long. However, I think my brother-n-law may be after my wife's yearly inheritance. I pray he is not that underhanded, but he has also mentioned my step-daughter's social security benifit 1 to many times. My question is; if I sign the conservatorship for my step-daughter, can he use that to take away from my infant daughter's inheritance? What should I do? My wife left no will and I want the inheritance and social security for both kids put in a trust fund.