would you be a sperm donor for you sisters' lesbian partner.  younger sister has a long term partner.  they are considering having a baby.  her partner would be the mother.  they want to pass on our family genes.  what legal responsibility would I have for the child?

in the state(s) you both live in.  

I heard a story on the radio, years ago, that a friend of a Lesbian couple donated his sperm. They had the baby and about 5 years later they separated and they sued him for child support and won. This poor sap did nothing but be a good friend and donate his sperm and now is on the hook until the child is 18

"Spare the rod, spoil the child", right?

Several states have made the sperm donor pay child support when the couple breaks up

a biological child of mine was  in the world unknown to me where I could not love, helps, protect him/her.

"Kansas sperm donor."  You better do your homework, and then some, unless paying child support for 18 years sounds like a good time.