It would be easy to think that you should name your sister AND her husband as guardians. The best practice is to name just your sister as the guardian.
If you name a couple as guardians and they get divorced after your death, you just unwittingly put your child in the middle of a custody dispute.
Naming only your sister as guardian does not preclude your sister’s husband from participating in the rearing of your child. In fact, after your death the couple can even go so far as to adopt your child.
Better to give your sister maximum flexibility to do what she thinks is best for your child.
Naming just one primary guardian preserves options your guardian will appreciate and prevents your child from being the unfortunate subject of a custody dispute.
However, for those (like myself) with a family situation where it just feels too awkward to name one but not both spouses as guardians, there’s a “trick” you can use. You can write your will to say something like this:
“In the event of my death, I nominate Susie Jones and Bill Jones to serve as joint legal guardians of my children so long as they are married to and not legally separated from each other. In the event of the divorce or legal separation of Susie Jones and Bill Jones, Susie Jones shall be the sole legal guardian of my children.”
If you designate a couple who are both close friends but not family members, there is really no choice but to name both, and that is perfectly fine. Just be aware of the risk you are taking if the couple divorces after your death.