There are many people involved here so you need to let everyone knows where the boundary is.
Your fiancé needs to know your daughter must be kept safe and not be touched physically and emotionally, your future son in law would need to know that she should be treated like another younger kid in school at least until they know how to set their own boundaries between them as they grow older.
Your own son needs to know when to step in for his own biological sister if things aren't right. You can't be there all the time but your son will likely play a real older brother role until your future son in law can be trusted (if ever) on that boundary.
Your daughter needs to know what is not acceptable and when to ask for help without feeling shame (although she also should learn not to taunt your future son in law and not to escalate situations, like all young women should learn as they grow up).
Your future son in law needs to know that you mean business as a father of a daughter, and know that even if you are also his future step father and she is a future step sister there should still be a boundary that even biological brother will not cross, and be respected. Do it firmly and with respect.
That's my own opinion. Hopefully one day your step son will know and respect you for being a father of a daughter, like how men respect each other when family is involved.
Your fiancé needs to know your daughter must be kept safe and not be touched physically and emotionally, your future son in law would need to know that she should be treated like another younger kid in school at least until they know how to set their own boundaries between them as they grow older.
Your own son needs to know when to step in for his own biological sister if things aren't right. You can't be there all the time but your son will likely play a real older brother role until your future son in law can be trusted (if ever) on that boundary.
Your daughter needs to know what is not acceptable and when to ask for help without feeling shame (although she also should learn not to taunt your future son in law and not to escalate situations, like all young women should learn as they grow up).
Your future son in law needs to know that you mean business as a father of a daughter, and know that even if you are also his future step father and she is a future step sister there should still be a boundary that even biological brother will not cross, and be respected. Do it firmly and with respect.
That's my own opinion. Hopefully one day your step son will know and respect you for being a father of a daughter, like how men respect each other when family is involved.