Some people do not have an awareness of money and how much they are (in my eyes anyway) throwing away every month on getting drunk every weekend on a night out, eating out all the time, takeaways, buying new clothes etc , updating your car every couple of years, and opting for fancier ones, getting nails, lashes hair extensions etc done very regularly, booking any event which half takes their fancy, and paying for the best tickets. Any money they have after essential bills are paid they just see as spare, which in their eyes equals "to spend" - on things that don't last.
They just don't seem to understand that you can use any spare money to invest in things for the future, either literally, to enable you to maybe retire earlier than them, or on things like your house, which would also increase the value, and your quality of life.
I've been in the position where someone I know well in very similar financial circumstances to me about 10 years ago, has made thinley veiled comments about me being stingy, because I have paused to assess whether something is worth the money while out shopping, for example, or intimated that I was somehow luckier than them as I have the option to retire earlier than them now (despite the fact that I've made very different financial choices over the years to allow me to be in this situation)
We spend money very differently but some people have a hard time understanding that if you spend on certain things, you're limiting yourself in other ways, and that's fine if you get a lot of enjoyment about the things you've spent the money on and are happy with your choices, but don't then go and suggest that someone is luckier than you because they have done things differently and now have more than you or they have an asset to show for it which will last and bring value over many future years.
It is VERY hard not to get wound up by people commenting in the same way as your inlaws, OP, but I've found it's best to just state calmly, "I guess everyone's different in how they choose to live their lives, spend money etc, aren't they? Would you like another drink/piece of cake/whatever?" to subtly change the subject to refuse to be reeled in. DO NOT tell them any details of costs, your income levels, savings levels etc as that would just invite further questions.
If your relatives ask you for a handout you just say any savings are invested or in inaccessible savings accounts to fund future goals eg kids uni fund/driving lessons or family trip of a lifetime in so many years. They wouldn't dare to ask you to take money out of savings that are meant for the kids and if they did I would tell them exactly what i thought of them.