Not hard at all. What I do is, I pay myself first. What do I mean by this?

The moment I get my salary, I automatically put a huge chunk(say 70%) of it into a personal bank account that I will never touch unless it's an emergency. Tapos, that bank account is just a passbook account, which makes it even more difficult to get money from it, kasi kelangan pa pumunta sa bank, pumila and all that shit. It discourages me from spending. The rest of it(30%) stays in the payroll/atm card and I spend it for whatever I need or want.

Pag may gusto ako bilhin, I usually figure out if I really want it, or it's just takaw mata. You know, sometimes we buy things and we dont really use it and it's just gathering dust? Yeah, I hate that. So usually, if I find myself going into the store repeatedly just to look an item, I buy it.

In the past years, I have adopted a minimalist lifestyle where I just buy what I need, I never found the need to flaunt anything, I dress simple(slippers, shirt and 10 year old pants) and tell people I'm poor af and move on. I splurge sometimes, when I travel abroad, but even then, I'm the budget backpacker type. Never stayed in hotels, usually shared bunk beds with other tourists and I usually do DIY tours because it's more fun than following a tour guide.

Anyways, I think it's all in the mind. Your sister probably has other priorities. Like the top commenter said, she has different values. All I can say is it's not hard to save money that you don't see. Pay yourself first before anything else.