I hope I'm a good MIL - I certainly love my DDIL to pieces - we hit it off straight away. My advice is much the same as anyone elses -
They are adults - they make their own decisions , I never comment on them I support them in all their choices..
I let them know I am always there if they need me for anything - help support, money (if i have it) and they only have to ask - I will always help if I can.
I regularly tell my DDIL I love her and she is the best thing that ever happened to my DS (it is true) (She will also regularly FB message me with 'love you xx' and it's lovely when a message from her pops up)
I listen to her when she needs a shoulder, and I listen to her advice when I need one (she is much more sensible and grown up than me in many ways)
Obviously I remember her birthday and often buy little things I think she might like at other times - just as I would my DD, she is in my thoughts as much as my DS and DD are, she is my family. I can't even bear to think of them ever splitting up - it would break my heart.
I am good friends with her Mum, who is a lovely person, and I accept that her Mum is exactly that, they have a very close relationship and if DDIL and DS decide to have children then I'm sure as most daughters would she will turn to her mum for help and advice far more than she would to me. But I'll always be willing and up for babysitting, and I bet me and her Mum will plan all sorts of double granny trips together 
In short I adore her - my DS really got lucky when they met, she has turned his life around in a way I never could have done - he was drifting and she gave him purpose.
I hope you have a great relationship with your DDIL - sounds as if you are doing a pretty good job 