Here, I am confused. I cannot say "younger daughter-in-law" as the order of sons is defined by their age but we can't say that about their wives. So shall I say "my second daughter-in-law"?
If you can't use her name (e.g. if you're introducing her), and need to specify exactly which daughter-in-law, I'd suggest "my younger son's wife".
Second daughter-in-law is ambiguous since you aren't clear on what is being counted. It's not clear which you mean: younger daughter-in-law, or wife of younger son, or wife of son who married second.