Bang "Now I'm having a meltdown again thinking it's unfair of me bringing another one into the world, I maybe am too old, do I want to do all the baby stuff again etc...AIBU to think it's cheeky for her to assume I'd babysit and assume my baby making days were over at the grand age of 34 and a half?"
I was 39 before I had my dd.
You are not old.
You are, however, far too interested in what your sister thinks. For a start, have as many kids as you like and can support, second only be a child minder if you want to be one, third, why does your sister assume you would do this for free!
"my sister doesn't want me to have another because she hates having old parents now, she think anyone over 30 should be put out to pasture" Please tell her to keep her offensive views to herself.
"She then text saying she hoped I'm not upset, she just doesn't see the point in me having a baby when she's due hers if I want a baby so much I can just give hers extra love and cuddles." I am surprised you continued listening after she gave you her views on your fertility.
"Trying to explain to her that that's her baby not mine, I want to experience pregnancy again, feel the kicks etc." *DO NOT bother explaining to her why you want another baby it is none of her business.
"I feel crap because she's really stuck the boot in about dd being pushed to one side for another kid!" Your sister is being abusive to you, she is attempting to affect your family in ways that are none of her fucking business. Please, please tell her if she ever speaks of this again you will simply pick up your bag and walk out/hang up the phone. If it happens a lot I would consider not seeing her much or talking to her much. Smile as she talks, let her words wash over you, she is bonkers.
"We never pushed her to one side but obviously a new baby did grab a lot of attention not to the point that my little girl suffered or felt left out though. It's bitchy of her and I really cba with her to argue about it," Then stop, don't do it.
"... just feel like if I do get pregnant my husband and I will be the only ones that are happy about it - which I suppose is ok I don't need her validation." Do not make your baby about her at all. It is none of her business.
Honestly, I would be removing myself from her company, she sounds awful.