Hi Yvonneho,
Don't worry. Just continue to encourage your girl to walk. When your girl is ready for walking, they're usually rather determined to practice, and soon after, they'll be walking. For my girl, when she was ready to walk, she practiced for days, and thereafter, was able to walk.
For now, you can put her on the stroller. I think the strategy to get them to sit on the stroller is to get moving, and not stop. That way, they have things to see, and won't be asking you to carry her.
When your baby is due in sept, your girl will be abt 1.5 years old, it'll be easier. For now, also try to talk to her more, so she can express herself more, and you can understand her better, to reduce frustration from not understanding what she wants. Also, get her to self feed. They may not hold the spoon well, but with practice, they'll soon be able to. My girl likes to use her hands, especially to eat fruits. So, next time, when you're busy with newborn, can give your girl her fav fruits or snack, put her on high chair, and that'll give you time to do things. I do that, so that I can cook dinner. Now, my girl is older, she'll play by herself.
Remember, small age gap has their advantages. I have a friend with small age gap, and they're playing so well together now.