I'm kinda confused about what you mean with "twin" sisters, but it may be like our sister mother/sister daughter thing (sis-mom/sis-dot). when the girls first joined, whatever member they clicked with well during the recruitment process choses them for their dot...and that's how initially we get them involved with everyone. then, about a week b/4 they are initiated, they write down their top 5 or so choices of who they want as a big...normally the new member coordinator just goes around and asks if the person can take them and that's how it goes. it's a fun family creating process. now my family is huge...i have 2 lils, one of my lils has 2 lils, and i now have a great grand lil which just means i am OLD...plus i have 2 dots and one grand dot...it gets kinda weird, but again, it's fun.
i think the "sis-mom/sis-dot" term is an alpha gam thing, but other sororities have variations of it (chi o has owl pals, sigma kappa has pearl pals, etc.) everyone does things somewhat differently, but this seems to work for us. and i think the only thing to make sure that the women are good bigs/lils is to remind them that sisterhood is very important, and the bond made within the big/lil sisterhood is supposed to strengthen that...it's a group within a group, and is something only shared with a few close people. ya know? you're all sisters...but not everyone is within your family. that's why it's special.
hope that at least made some sense...
monica