How to make a 3.5-year-old stop deliberately waking up her 1-year-old sister?

Question

We've got two daughters of 3.5 (B) and 1 (L) years, respectively. B's behaviour is sometimes a bit reminiscent of what was described in someone else's earlier question.

The particular issue/situation that I would like to inquire about is that, for the past six months, virtually every time L is on a good way to enter one of her afternoon naps¹, just when she seems like she's falling asleep, B will deliberately make some noise to wake her up.

Similarly to the linked question, I suspect B sometimes resents L for taking away everyone's focus on her. This is what makes this situation particularly frustrating to us - L's nap (can be one or two hours) would be the one time where B could get our full attention. I'm personally looking forward to playing with B², but somehow, B manages to sabotage that opportunity herself. As a result, L is awake again, and she'll take up a part of our attention again.

My question, thus, is two-fold:

• Why does B do this and how do we encourage her to stop it? I'd like to let B notice the advantage of having our attention while L is napping, but B prevents that from happening in the first place.
• How should we react right when the issue occurs? We have tried some ways of stopping the noise (most straightforwardly, telling her to be quiet for a moment, or firmly covering her mouth with a hand ...), but that just makes her playfully find other ways to keep up the noise level (humming loudly, clapping, ...).

Some more data points:

• If the children are in the same room when this happens, this plays out as described, and B is successful in her quest before we can bring L out of the room.
• If we bring L elsewhere in time, but B realizes L is about to sleep, B will start walking through the house, yelling "L, wheere aaare youuuu?" ... or B will immediately state she's tired, as well, and wants to sleep where L sleeps, promising to be quiet. Either, we grant that wish, and she'll noisily join L, waking her up, or we deny it, in which case B will throw a noisy tantrum, waking L up even through a couple of doors/walls.
• When we ask B why she does what she does, she'll state "L isn't allowed to sleep." ("... isn't allowed to ...", without any further explanation, is her favourite complaint when she doesn't like something the way it is, anyway.)

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¹: It's somewhat hard to achieve, she easily feels disturbed by noises, and if the attempt for a nap fails, she might easily stay awake throughout the day, ending up cranky in the evening. At the same time, spending 20 to 30 minutes to make L sleep several times a day eats away non-negligibly from our time, as well.

²: Admittedly also because, well, toys compatible with a 3.5 year old are still a bit more interesting for an adult than those compatible with a 1 year old.

Answer 1

One thing to try, would be to enlist B's help in putting L to bed. Talk to B about the fun stuff you are going to do once the baby is sleeping and ask her to help you put L to sleep as quickly as possible. It will take some trial and error to figure out if B can do some quiet activity alone in another room, or if she needs to be with you while L goes to sleep, but at least B will know where you and L are and have a reason to be quiet.

Answer 2

Children tend to do more of whatever gains them attention. This holds true for negative attention as well. As a general rule, give more attention to the behavior you want to reinforce. For every scolding, try to find at least five opportunities to praise a desired behavior.

Make plans with B in advance. You're saying she prevents herself from discovering the advantage of a sleeping sibling, well, tell her straight up. "It's soon time for L to have a nap, do you know what we can do then? I would really like it if we could play with your such and such toys. Would you like that? We just need to cooperate so that L can fall asleep". Obviously then execute such plans if given the opportunity.

Playact overly expressive whispering or sneaking, to make peace and quiet a fun game.

Use a personal language. Don't make your playtime or attention an earned treat, let her feel that you want to play with her if enabled. Don't tell her to be quiet, let her know you're sad that you're missing out on the opportunity to play. Children want to satisfy, if they can. They understand emotions. They don't want to follow arbitrary rules. They generally don't understand why they can't do X.

I read that you've been experimenting with time outs. To me, that raises warning flags that you're making this a matter of obedience. I think you'll have vastly better chance if you can turn this into something B can value in itself. As a positive side effect, I believe you'll grow a closer relation to your kid.