1. Revise her bed time routine, 7:30-8:00 could be a bit too late for her sleep cycle to be already pass, try moving it a bit earlier by 15-30 minutes earlier, and try this for a week.
2. The strategy of sitting next to her is good, however you should try to avoid the contact o keep it to a minimum.
3. Your Bed-time routine should be short max 30 minutes.
4. Wait a few minutes (10mins ideal) before attending, sometimes we are so scared of letting them make some noise with the fear of waking up the siblings but by giving time we give them the chance to resettle again. If there is not chance there is not attempt.
And yes it is possible that in the beginning the brother will wake up but in not more than 3 nights both will be used to, one to keep sleeping through the noise and the other one to resettle.
If you don’t feel like testing your luck, then move the good sleeper baby to your bedroom in a portable cot for a short period of time and start the strategy! 3+ nights of hard work on the same method will be great to see changes.
5. Do not bring her to your bed, children love the company of an adult and will develop the habit of crying every night to get moved into the parent’s bed. Do it one more night and once you are ready talk to her about that won’t be more visits to the bed and explain that she will need to resettle in her own cot.
6. Once you’re are 100% convinced that is the time to start. Begin to apply the tips 1-5 and do not mover into your bed again. Hard Work? Yes but no more than what you do now and the good thing is that this will have positive results and more happy long nights. Give it a go for a least 1 full week (5-7days) and then decide if works or not.