C.O.
S.:
You are NOT dumb!!! This has been asked MANY times!! And it's okay!!
You are right - MIL, SIL, BIL, FIL - all mom, sister (or son), brother, father in law.
OP - original poster
TIA - thanks in advance
HTH - hope that or this helps
DD - Dear daughter
DH - dear husband
SD - step daughter
SS - step son
BF - breast feeding or best friend (depends on context!! LOL!!)
LO - Little One
GF - girlfriend
SO - significant other
There are more let me get some caffeine in me and I'll do more!!
EDIT: Thanks AV!!! I knew I was forgetting some!! I needed to get some caffeine in me first!!!