Both terms tend to have wildly varying connotations, depending on who you ask and in what setting they are used. This is true even by the people that claim them as identities.
For some people, femboy and sissy are interchangeable in meaning whereas for others sissy is a slur and femboy is value neutral. Also, in many NSFW contexts, both femboy and sissy have become fairly synonymous with fetish fashion, submission, humiliation and forced feminization. Outside of queer spaces, femboy has historically been used as an epithet to demean or humiliate in much the same way as sissy or tranny.
Sissies generally seek domination whereas femboys don't necessarily seek domination. Also, sissies are like femboys in drag--in other words, they overdo their femininity and their appearance of vulnerability. They might also present in a more childish manner.
Sissies tend to derive gratiffication in roleplay that has to do with themes of being forced, being "emasculated" or performing servitude (maid, slave), as well as physical or mental masochism (or both), for example being called terms such as bitch or fag, being spanked or gagged, cuckolding, being "bimbofied" and/or being dressed in over the top "girly" outfits usually considered funny or ridiculous by current standards.
The sissy kink also overlaps with chastity play. Sometimes sissyfication also eroticizes aspects of female physiology such as menstruation or pregnancy, as a sign of the sissies being "robbed" or driven away of their "manhood" (which also may or may not involve being degraded for it).
A femboy is a male (usually young, hence boy in the label) who is "gender atypical", having a mix of appearance, behavIour and personality that aligns more to that which is expected on girls and women.
A femboy would be a synonym of the first (older) use of sissy but not of the second. With the second there is an overlap, since you can have femboys that are kink sissies and femboys who are not.
My goals make it very clear tha I am a sissy.
To be a dainty & delicate little girl sissy in the way I think, act and present myself
To completely accept female control and never question what I am told to do
Proud to show everyone what a precious little girl and Pretty princess I am
Love Sissy Princess Annie
