D for daughter, S for son or sister, H for husband, and a D before them stands for anything beginning with D that is appropriate - dear, darling, dratted, damned - depending on what they have done.
SiL is son-inlaw or sister-in-law. Some others are obvious, but there is a list of acronyms that you reach by selecting the "acronym" link at the right-hand-nde of the coloured strip at the top of to post, jsut after "Lat Day" and FAQs".
Of course, sometimes people make up their own abbreviations as they go along - like the rest of us, you just have to guess at what those mean!