I know a family which has this sort of setting. The elder brother got married and then the younger brother started liking the younger sister of his SIL and then they got married. It worked out pretty well for them. Actually, there's an interesting story to this as well. The elder sister had a miscarriage and doctors said that they won't be able to have a baby ever. But when, the younger brother and his wife had a baby, they gave the baby to the older couple(I'm using older relatively. They're all in their 30s). They live in a joint family.. so the younger couple (who had 2 more kids later) can see their oldest kid growing up in front of them, but the older kid recognizes the older couple as her parents.

how to approach my sister-in-law’s sister.

You could do that directly with the sister. Or just the transmit the message through an appropriate chain. Share your intention to your brother/bhabhi and then they can take it up.