Your stepfather who married your mother a fully consummated marriage is a Mahram for you and you don't need to wear Hijab in front of him. Same is his father.
Family unity is of vital importance. It sounds like there may be other issues here - more than that of halal food - possibly issues of balance of power between your parents. Why would your mother not want to be with your father? Is your father respected in the family? Do you have a close bond with him?
You should avoid hurting or disrespecting any of your parents. Even you have no way but to chose one option of your parents different options, you must keep their respect and nice to both of them in every possible way.
Milk daughter is not allowed to marry with her milk father nor her milk brother, but she does not have a share in inheritance like a real daughter unless by a will and within the limit of the one third. As she is living with your family due to her mothers passing away, you and all your family members should give her all moral support and look after her in every possible way.
Wassalam.
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