Good question, although it really depends on how your kids' personalities are.
for an SIL, I'd want someone that is caring, modest, not arrogant, morally sound, religious, kind-hearted, understanding, loving to my daughter, well educated and on the road to being financially stable.
For a DIL, I'd want someone that is also caring, modest, moral/religious, well educated, but also sensible and not one of those high-maintenance types (they annoy me to no end), not possessive over my son, not dramatic, and cooperative. I wouldn't want her to live with me though. I would also hope my own daughter is all of those things- i.e. i would teach her to not be possessive, dramatic and high-maint.