Should I adopt two new kittens?

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Today I went to my local pet store to get some supplies for the cats we currently have at our house. This pet store had an adoption booth set up with so many kittens needing homes. My boyfriend and I became smitten with two lynx siamese kittens ( a brother and sister ) pair. The note on their cage said they needed to be adopted as a pair so we can't separate them. Not that I even want to! So here's the problem. My boyfriend and I live in a small two bedroom house, which is really small, more like a one bedroom house with a big closet, and we already have two cats. We are great kitty parents and would spoil these new cats rotten, but is it the right thing to do? Are we maybe being cruel to these cats by bringing them into our small home? Under no circumstances could these kitties be outdoor/indoor cats because we live by a busy street. Thoughts and opinions most appreciated.