Lightweight step-thru frame?

There is this:

https://www.planetx.co.uk/i/q/FRHOLAQW/holdsworth-la-quelda-womens-steel...

requires

https://www.planetx.co.uk/i/q/FSSXCDCH/sunxcd-sxcajust-chain-tug-with-re...

you are looking at 11-12kg with derailler, mudguards, average wheels and sensible tires.

There is also this:

https://www.genesisbikes.co.uk/bikes/womens/womens/womens/columbia-rd

but 14kg! mostly due to the choice of a geared hub and too wide rims/tires.

I built up this for my Other Half using a carbon step-through touring frame from High on Bikes on Ebay:

It's been a good bike for her, around 8kg and not even expensive thanks to the old parts bin. Long wheelbase so nice and stable. The fork was one I had. Her choice of pedal colour

How about the Charge Bikes Grater Mixte http://www.chargebikes.com/grater-mixte-2/

Much lighter than most of the bikes mentioned above. The version in the link is hub geared, but there's also a standard version 

OP is looking for a reasonably light bike for his inseam of 33". That's about the same as mine so I guess he is looking at a 22" or 56cm frame. That's an unusually large size for a ladies frame which is why I suggested the Bob Jackson option. That will get a step through frame of a sensible weight. Other frame builders are, of course available.

I brought my mum to a WheelsforWellbeing session at Croydon today - trying to get her to get back into cycling after she put a bit of weight on. She complains that the folder she has is too uncomfortable and I wanted to see if she'd get along with trikes or recumbent. They had the v basic BTwin step through bikes there. I'll be ordering one of these:

https://www.decathlon.co.uk/elops-900-step-over-classic-bike-alloy-grey-...

The step through was about as low as you could possibly go, which she loved... the weight seems to be 15kg though, perhaps you could shed a couple of kilos somehow?

A while ago, I'd have said Decathlon, but not anymore. Their quality has dropped off a cliff and is shockingly bad.

Have you looked at the Decatholon step throughs - They do a aluminium version 

https://www.decathlon.co.uk/elops-900-step-over-classic-bike-alloy-grey-...

It's 15kg but it has everything on board - a few removals and upgrades may reduce the weight.

Its £329

That Whyte bike looks great. 13kgs to start with (ish). 

You'd loose 1/2 a kilo from changing the tyres out (720grams each!)

God knows how much the wheels weight but I bet they there's another 3/4 of a kilo to easily be lost there.

12kg bike for the sake of £300 upgrades.

Looks great, I might actually be tempted to get one for my mrs, that'd suit her really well. 

Check this out.

http://www.bobjacksoncycles.co.uk/product/open-ladies-frame/

Sizes all the way up to 25" (63.5cm).

This looks like a good list to start exploring from

https://www.cyclingabout.com/a-complete-list-of-womens-touring-bikes-ste...

Not sure if the Large frame size will be big enough but it is worth looking at the Whyte Strawberry Hill, one of the lightest I found when looking for my a bike for my wife

https://www.whyte.bike/strawberryhill/

How does a dropper post aid getting your leg over the top tube of the frame in the first instance?
It also adds a lot of weight and a heap of cash for a decent one

another option is a hybrid bike with a dropper post to lower the seat for getting on

I think you might have a little bit of trouble finding a reasonably light step through bike. There are a couple reasons for this; usually, they are aimed at more "casual" cyclists, at a lower price point, hence heavier components. Also, the frame needs to be beefed up a little to stop it being all floppy and able to cope with how the frame deals with forces differently (losing the triple triangle frame means bending happens in different places etc).

I'll try and be a bit more helpful now!

Both Brompton and Moulton might fulfil your needs for a really easy step-through frame. Small wheels don't mean less stability as such https://janheine.wordpress.com/2011/04/27/bicycle-stability-everything-w... - though I'm not denying small wheels can be "twitchier" as they are more responsive. The trouble with Moulton and Brompton is they cost a huge amount and they require suspension to not make the ride harsh. You could even take a leaf out of Sheldon's book and convert an old Raleigh Twenty https://www.sheldonbrown.com/org/raleigh-twenty/

I think the only other option you have is to get a reasonably light Alu step-through frame and build it up with decent lightweight components.

Do you want new or used/retro, what's your budget, what do you want gearing wise 5/6/7 or 8/9/10, how low do you require the 'step thru' to be, full 'dutch loop' or something like this that is just ending https://www.ebay.co.uk/itm/LADIES-TREK-BIKE/173317119426?hash=item285a81...

Given your 33" inseam how high you can actually get your leg over is important in terms of which frames/bikes might be suitable.

My mum who is about 5ft 1 but a reasonably athletic 68 rode a XS hybrid mens frame ( Mongoose Crossway 200), most frames with a very low step will be ladies but a small hybrid frame with an extra long seatpost and upangled swan neck stem or high angled/adjustable ahead stem  could suit. Or if you don't mind an ugly stem convertor you could use a high angled ahead stem on an older frame, again depends on what you can manage to get over.

There are a few older frames that are low down in the stepover class that aren'r ridiculously heavy, the weight differential is mostly in the wheels/tyres and rest of the components as opposed to the frame itself but I bought a lady friend a Bottechia steel loop frame, it was ridiculously light but rare as rocking horse poop. Here's a few examples of R531road frames that might be suitable

https://www.ebay.co.uk/itm/80s-British-Eagle-Randonee-Ladies-Road-Tourin...

https://www.ebay.co.uk/itm/Vintage-Ladies-Raleigh-Ventura-531-Reynolds-R...

https://www.ebay.co.uk/itm/Vintage-Gitane-womans-racing-bike-not-Peugeot...

Obviously the smaller the frame the lighter it will be AND the lower the step through height, it's basically what the pros do, silly small frame and whack a massively long seatpost and stem on it so it looks like a kids bike under them.

If new then that's a whole different ball game as light isn't cheap to come by relative to an older/used bike/frame. 

Look at ladies shopping bikes. Usually alloy frame and big wheels.

PS there no such thing as ladies/men's bike only size difference. I can ride my wife's shopping bike