Family signs in ASL are built on one rule, and once you know it, you can almost guess the signs you have not learned yet. Female signs are made at the chin. Male signs are made at the forehead. The handshape and movement carry the relationship; the location carries the gender.
The rule comes from old-fashioned headwear: the forehead was where a man's hat brim sat, the chin was where a woman's bonnet string tied. Nobody wears those anymore, but the signs still remember.
Mother and father
Mother is an open hand with the thumb tapping the chin. Father is the same open hand with the thumb tapping the forehead. Same shape, same tap, different location. Parents touches both: forehead then chin, or chin then forehead.
Grandparents
Grandmother starts as MOTHER at the chin and bounces forward in two small arcs, one generation out. Grandfather does the same from the forehead. Think of the bounce as stepping back a generation.
Sister and brother
Sister starts with an L handshape at the chin (the female spot) and brings it down to rest on the other index finger. Brother starts with the L at the forehead and does the same. The second half of both signs is the sign for SAME, so sister literally reads as "female, same" and brother as "male, same."
Aunt, uncle, cousin
Aunt shakes an A handshape near the chin. Uncle shakes a U near the forehead. Cousin shakes a C at the side of the head; signing it near the chin or forehead can specify a female or male cousin.
Niece, nephew, twins
Niece twists an N near the chin. Nephew twists an N near the forehead. Twins touches a T to one side of the chin and then the other.
Baby, child, family
Baby rocks your arms as if holding one. Child pats the air at a child's height; patting twice at two heights means children. Family starts with two F handshapes touching and circles them around to meet again, the same shape as GROUP and CLASS with an F for family.
Talking about your family
A typical first exchange: YOU HAVE SISTER BROTHER? with raised eyebrows. Answer: I HAVE SISTER TWO, BROTHER ONE. Numbers come after the noun. To say how old someone is, sign the relative, then OLD and the number from the chin: MY SISTER OLD 12.
You will also want married, husband, and wife from the Relationships and People categories, which follow the same chin-and-forehead logic.
Practice
Sign your own family tree out loud, one person at a time: MOTHER, FATHER, SISTER, and so on, pointing to a spot in the air for each. Then add their names by fingerspelling. That is both a vocabulary drill and the pronoun setup you learned in Pronouns.
Next: Feelings so you can say how everyone in that family is doing.