Reuven, Shimon and Levi are brothers

Reuven is married to Rachel and Leah. Rachel is Shimon’s daughter, i.e. an erva of Shimon

Levi is married to Miriam

Reuven dies, shimon can’t perform yibum as rachel is his daughter an erva and can’t do yibum with leah, a cowife so levi does yibum with leah (but can do yibum with either)

levi then dies… shimon can then not do yibum with leah, who was the cowife of his daughter but can also not do yibum with miriam who is the cowife of the cowife

reuven and shimon are half brothers through their father, when shimon dies, reuven cannot take miriam in yibum

mothers sister

two half brothers, reuven married the sister of his fathers second wife if reuven then dies, shimon cannot take reuvens wife in yibum and she his is erva