Agree with Scaramouche.
Do not suggest it - ever. Here's why:
1. Sisters are jealous, for good reason. There are many real-life stories of younger sisters (or a hot chick's "best friends") trying to steal her man. (Yes, it's a popular porn theme as well, but there's a reason for that.)
2. She may not want to share... no not your dick, your wallet.
3. Obviously, the likely result (from your perspective) is that this will lead to threesomes - because - THREESOMES with SISTERS!! But your SB may not be comfortable being a whore in front of her sisters, or seeing her sister as a whore, or - even worse - she may not be bi-sexual.
4. Fear of losing her SB to a younger, hotter woman. All SB's have self-confidence issues. She may be very worried that if she serves up her sister to you, she will soon be left behind.
So as mentioned, let HER bring it up. If she does, then you can "reluctantly" agree to meet the sister, then reluctantly agree to a date with her. The point is to let your SB pull you along into heaven. And if she never goes there, you won't step on any landmines.
Life is good
The Cat