YY I agree that you need to evidence things, rather than just gush about how great they are.
Also
Don't say anything about them " deserving " to get a child , SW don't like the implication that adopted children are a consolation prize for those not lucky enough to conceive.
Don't say how desperate they are to have children, even if they are.
Don't say anything that implies you have a clear idea about the type of child they should adopt eg " our whole family are looking forward to welcoming a new baby " " John is looking forward to playing football with his new son "
You need to talk about what your sister and BIL have to offer a child /children who have had a difficult start in life. Do talk about their strengthsand their skills , the problems they have faced in life and how they have over come them.
Eg redundancy, family illness, death of a parent or grandparent
You want to show their coping skills, how they face problems together in a constructive way . How their skills/personalities complement each other . How they use the support of family /friends /community groups etc
Don't pretend that your sister and BIL have had a perfect life /marriage - it's not plausible and it's not what SW are looking for. They will be adopting a child who has had problems in life and will probably have more -they need families who understand this, who know that it's NOT the same as raising kids who are born to you . Who can use their life experience and skills to help their child and , if necessary, advocate for them .