Congratulations, Heather! We are finally getting the opportunity to try and socialize Toto with other animals and it's very difficult. She was 2 in July so perhaps it's her age and the fact that she is a spoiled "Angel Brat"!!
We are actually hoping to adopt a little rescue but she won't be ready for quite a while. Meantime ... her sweet Foster Mom and I are are working with them both and maybe with the help of a bunch of Yorkies & a sweet Poodle we can make progress. So far Toto acts like "Miss Snotski" and won't play with anyone but we are going to try some "play dates" with maybe one-on-one and see if they will interact!
From what I am observing with Toto, it's much better to let them do this on their own, although I do get the urge to shove her in the group when they are all playing and she's sitting there all by herself! We did make a little progress last time by taking a few of her toys along and encouraged everyone to play and she actually did a little! I would say that it probably won't happen overnight but you have a great opportunity by getting a baby and especially a little half sister! Good luck and please let us know your decision and how it goes?