Well yeah, he’s a dick. Or at the very least his delivery and communication skills are utter crap. But looking at each part…
He's talked about my sister being in great shape, and that it's great how she works out so much. (For comparison - I am a normal weight - but definitely not toned!)
This is vile, obviously. If he’s so enamoured of her maybe he should have dated her instead. 🙄 He’s clearly trying to influence you to be more like her.
He often says it's important in marriage to keep in shape and 'not let yourself go'. He says this a lot. He says it's not surprising if someone finds a partner less attractive after they put on a lot of weight.
This is reasonable imo, though he doesn’t need to harp on about it.
He told me he doesn't like the jeans I wear, and thinks I look better in other things (he compliments me a lot when i wear skirts and dresses, or dress up). I didn't wear those jeans again, thinking maybe he had a point. (I found other jeans which he likes better).
This depends on the individual. Personally I don’t mind a partner expressing some likes within reason; I have my own preferences (hate graphic tees on men, for example). Some women don’t like their partners to express any kind of thoughts/opinions about their appearance. It depends on where you fall on the spectrum. I will say though that many men seem to dislike the baggy mom/teenage boy style jeans, in case those were the ones in question.
He said that when I go to visit his family, he 'wants me looking my absolute best, wearing the very best coats'.
He has mentioned how women in his family look after their appearance - like his sister in law (who is very thin and exercises a lot) and his mum (a normal weight, buys a lot of nice clothes).
This sounds like a background/class difference to me. It’s up to you if you want to go along with it or ignore it. He’s probably more concerned about it than they are. If you feel your clothes aren’t up to snuff then obviously he should treat you to a shopping trip.
He compared these women to my mum, who in the past has struggled with her weight. He said she 'doesn't give a shit about her appearance. Look at my mum, then look at yours'. (he was saying this in the context of being critical of my mum about something else).
This is just unpleasant and uncalled for. Regardless of what he thinks, he can keep his mouth shut and have some bloody manners.
He has told me he thinks I need to buy better clothes, and that he doesn't like a jumper that I love wearing. He thinks it's unflattering (it's loose with drapey arms - but I think it's stylish!)
Again, take him up on this and let him pay for it. It’s only fair, after all.
It sounds like he wants to shape you into someone more in line with his family. The question is, what do YOU want? You’re completely entitled to ignore him, or to tell him to piss off. Regardless, I would be pulling him up on the rudeness. There’s a difference between hinting that he likes you in dresses and insulting your mother, for god’s sake.