Since Jill's partner is the sibling of Jack's best friend and Mike has only one sister, Rachel, it means that Jill's partner is Rachel. Therefore, the relationship between Jack and the spouse of his sibling Jill is brother-in-law to sister-in-law, as Rachel is Jack's best friend's sister who is married to his sister Jill.

Laura, being Jack's spouse, is related to Jill by marriage as a sister-in-law. Following the family connections, Laura's relationship to Jill's spouse Rachel is also that of sister-in-law, as Rachel is married to Laura's husband's sister.
In a family, there are two siblings, Jack and Jill. They are both married. Jack's spouse is Laura. Jill's partner is someone else's sibling - specifically, the sibling of the person Jack considers his best friend. Jack's best friend is named Mike, who has only one sister, named Rachel. Describe the relationship between Jack and the spouse of his sibling Jill. Additionally, clarify the relationship Laura has with Jill's spouse.

Respond very briefly.