H.B.
My two kids are just 17 months apart and we never experienced this when my daughter was born, so I wouldn't blame it on the closeness of age of your daughter to her younger sister. I think it's just something toddlers do, and if they can get you to respond by changing your routine with them, they'll just keep it up to see how far it can go.
I think the most important thing for bedtime with toddlers (and babies for that matter) is consistency and firmness. We keep bedtime fun and enjoyable too, but we stick to the EXACT same routine every night, and don't alter it because of crying or protests. Because of our firmness, any protests don't last long, and are not extreme. I also think it's important at this age that your daughter be falling asleep on her own, not with you rocking her. Our routine is to put on pjs, brush teeth, read a few stories, bedtime hugs and kisses, then tuck into bed. We pray with each child once they are in bed, for my son I talk a little bit about his day and what things he's thankful for (he's 3.5) and we pray together, and with my daughter (2) my husband or I (whoever is putting her down, that varies each night so they don't expect that it is always me) pray and then sing her a few songs. Then we go out, shut the door, and that's that. My son will usually get up for a potty visit or two, we make very little issue of it, just say, "Ok, tuck yourself in again when you're done" so that it is as unexciting as possible. If my daughter calls out to us or cries, we sometimes just let her settle herself back down (she's still in a crib) or if we do go in to her, we are firm and unemotional, don't pick her up, and just say "it's sleep time now" and lay her back down with her special blanket and go out. She sometimes undresses herself, so if that's the case I'll put her pjs back on her quickly and without making a big deal about it, and then lay her back down.
I think if you and your husband just stick with the same thing each night and don't give in to any protests, she'll settle back into her routine quickly. As much as you love her and want to make bedtime pleasant, the most unpleasant situations usually result when the kids call the shots, not the parents. As a loving parent who knows best for your daughter you have to be firm in the face of her cries, sleep is what she needs most at nap and bedtime, and she'll accept and enjoy it if you don't make any exceptions.
Good luck, sleep problems can be such a bear to resolve! I have used and highly recommend the book Healthy Sleep Habits Happy Child.