They are your first cousins by marriage.
If you and another person have parents who are first cousins, then you and that other person are second cousins, because you have a great-grandparent in common.
Okay lets go in code. You are person Z and your relation is person Y. Sisters A and B have children who are first cousins to each other (one of these is Person Zs mum). These first cousins have children who are second cousins to each other. One of these second cousins is Person Z. The other cousin has a child. This child is Person Y. Person Y and Person Z are second cousins once removed (the once removed shows that there is new blood so to speak in the family). If person Z (you) were to have children, they would be th ird cousins with person Y. Does that make sense?
No. Step cousins are not related to anyone in your family, other than perhaps your step-siblings or step-parent.
Your are related to some of your first cousin's aunts and not to others. The aunts of your first cousin to whom you may be related are your mother and her sisters or your father's sisters. One of the parents of your first cousin isn't related to you, but married into the family. Anyone of your cousins aunts who are related to that person would not be related to you.
Third cousins.
This is just my opinion. Even if your step cousins you are still family. So it's kind of weird to me.
Most people would expect cousins to like each other, since they are members of the same family. That does not mean they always do.
You are related to some of your cousin's cousins, and not to others. If you have an ancestor in common, you are related. If you do not can a common ancestor somewhere, then you are not related.
Your question is a little unclear, but I presume that you meant to ask if cousins could get married. Some states allow it, some do not.
Quite possibly not at all as your cousins' grandmother need be no blood relation to you at all on one side of their family. At the other extreme it could be you or your brother, or you could be a first cousin. Plus lots of other possibilities - for instance, your brother could have married your cousin's grandmother's daughter, making the person your nephew.
If your talking bout your close family then 4 or 5 should be good. If your talking about other family like, aunts, uncles, cousins, etc then it could vary greatly. I have 5 people in my family and 6 or 7 cousins so.
Your niece's child and your grandson's child (your great grandchild) are second cousins, once removed, to each other.