Cash is King. Whether £10 or £1000 every little helps. Add what you can afford and thats it, you are being thoughtful because you are giving something they have asked for. Then afterwards dont think about it after that. If she complains then just tell her that's what you can afford. Smile and go elsewhere if you can or change the subject.
Ah, that age old thing that some people planning their wedding day think their event is the most important thing to all their guests too. Well nobody really cares tbh.
Your sister sounds like hard work. Well done for being bigger tbh and actually going. Take your buggy, your envelope and cash, eat, smile, leave.
We received cash for our wedding and stil have most of it stored in our account, it's always helped us out being in there.
We received 3 photo frames, all of which i have given to charity.
I wouldnt listen much to your sister, she sounds quite naive and clueless. Stand confident, tell her exactly what she does not want to hear, that of course you will be bringing the buggy and her niece/nephew will be there for the day as you think it would be lovely with your parents there too and sharing the day together. Being really nice (behind gritted teeth) is the worst thing you can do in this situation.