as i said in my post in bipolar, i agree that you must take care of yourself first. reaching out to her husband and daughter may be healing to you as well, but i believe that when that time comes you will know. there is no time limit. we are all different. from all you have said, i can tell she is like a sister to you. i have a friend i consider a soul sister. it sounds like something of the kind.
do not compare your suffering to that of her husband and daughter. comparing suffering is not useful to anyone. if you try to help someone walk when you cannot stand, you will both fall faster.
listen to your heart and your gut. your heart will want to reach out, especially to the daughter. your gut will tend to be fearful, and the fear is to keep you safe. at some
point, the reaching out will
be safe for you, and your heart and gut will agree
on what to do.
i am glad you made the attempt to drive by the house. you tried. you might have felt differently, and if you had, you might have stopped. it is ok that you did not stop. it means you are not ready.
one thing i could suggest is... because her house is full of memories... to consider arranging to meet up with her spouse and daughter - or even just her daughter, in a different place. It could be your home, or a neutral place like a coffee shop. Although since you guys might get teary together, maybe a more private place would be best. Either your home, or perhaps a park if there are areas which afford some privacy. again this is just something to think about.
you could also think about sending them the occasional card or a note through the mail. it doesn't have to say much. you could just say "thinking of you. i miss her too." or something. or, you might consider initiating correspondence with the daughter through email... if you want. email can be helpful because you don't have to be right there right then. you can answer it at your leisure. and you can edit things. you can cry in the middle, save in drafts and come back to it later. it is less intense than face to face or even phone. but not saying you ought to do any of these things. just tossing out ideas, in case any of them might feel "OK" to you.
i heard that you want to do something, and feel some responsibility for your friend's daughter... i think i would too, even if technically you have no such responsibility. this is a feeling that is an extension of the bond with your friend. remember this is a feeling. it is ok for it to stay with you, just as a feeling. again, you can act on it when you are ready.
remember that what you do or do not is not right or wrong. there is no "right" way to grieve. do what you feel is best for you, and is healthiest for you.
i can't imagine losing my sister-like friend the way you did yours. she is far away and we can seldom talk these days but i know she is there, and if she were not, i would feel so lost in the world. i am sending you lots of hugs, and warm thoughts.
~ waves ~