Go for the two weeks and be glad they still want you to visit.
I understand your concern if your DIL was drinking to excess, but frankly if I had been her, I would not be inviting you back!
What she does in her own home is not your business, is it?
And no-one will stop drinking because her mother-in-law mentions it, so please think hard about how you will react if she drinks during your next visit.
In my childhood my maternal grandmother spend the eight weeks of her school holiday, she was a teacher, visiting us, as we lived in Scotland and she lived in Denmark. Then we didn't see her again until next year.
Three times in the 14 years of my childhood (she died when I was 14) we went to her for the fortnight of my father's annual summer holiday. And twice for truly memorable visits she came at Christmas for ten days.
That was in the 1960s, and I sincerely doubt anyone would pay such long visits now. I would not want a visitor staying here for three weeks, or longer, however much I liked the person.