Only your SIL can really make that call because she knows her child and if the child is easy to take along or high maintenance.
Our first family trip my sons were 18 months and 3 years old. My younger son did great and had lots of fun, but my older son would not have done great at 18 months (he started the terrible 2's really early).

We rode most of the rides in the Magic Kingdom with him, but none of the roller coasters.
Reasons to take the toddler
- The characters LOVE toddlers!! They all spent a lot of extra time with my DS. I would usually say, come on and let the next person have a turn and the characters would motion "no" and want to hold him and play with him longer. We've been twice since and the characters spent a lot more time with him when he was younger.
- The pictures are priceless!
- Everyone else can enjoy seeing the Disney magic through the child's eyes.
- The toddler is free until they are 3.
Reasons not to take the toddler
- If you want to keep up with everyone else in the group and don't want to miss out on riding any rides.
- If you want to spend time with extended family and don't want to leave meals early or skip meals to let the baby have a break.
- If your toddler does not like being confined to a stroller.
- If your toddler doesn't adjust well to change and can't sleep in a pack-n-play.
- If your toddler is afraid of characters in costume. How did the toddler do on Halloween? Take the child to Chuck E Cheese and see if he is afraid of Chuck E.
- If no one else in the group will be willing to help or hang out with her and she will be isolated for a week with the toddler.