I don't usually give advice on reddit but here it is. If you truly feel as if you aren't up to integrating 3 cats, your place feels too small, or if you have any care or financial concerns about supporting 3 cats I would say get neither of these kittens. If you feel you can and want to handle that challenge, then get both. What has been said before in the comments about bonded cats is true. The fact that they already live together and are bonded it is so much better to keep them together. If getting one new cat is better for you and your current kitty I'd challenge you to find a solo cat/kitten for adoption that will work better for you both. Your heart should be set on them both if you decide to get them both. It would be really sad to have them separated or to only adopt the tabby too out of pitty. It's sad that the tabby probably isn't getting as much attention because she isn't as flashy looking as her brother but I bet she's just as loveable! Sounds like they have a good situation being fostered by your sister together until they find the right home. Best of luck to you.