OK Prudie there is much more here than you or anyone else has addressed...

First (as the 'girl' mentioned) she is supposedly 21 and living at home. This girl needs to wake up and realize that she is a woman now (if in fact she is 21). As an adult, the young woman needs to approach her mother as such and yes apologize for reading the letter, as sealed letters no matter where they are, belongs to the addressee.

Second: the 'cat is out of the bag' so they should discuss the child placed up for adoption and clear the air.

Third: if the daughter and mother don't get along so well (why is a 21 year-old still living at home in such a relationship?) this is the time to start being transparent, mend fences and get on with life!

Four: What happened to Dad? I would think that the daughter would be more interested in knowing what, how and where her true father is??! Yes he may have drunk a lot 20 some odd years ago, but maybe life is/was different back then and he is now a better man! At least find out. He may be a great guy now with a loving family and the daughters may have step-siblings who, quite frankly are just as closely related as their mother's child out of wedlock (unless the father was also the girls' father).

This family needs a lot of help and I suspect that outside counseling may be in order.