If BIL is coming along he will not be comfortable making you spend on their sight seeing I feel. You can tell them that you will spend for everything and later you can divide that amount by 2 or 3 (depends on whether you want your other sister to pay )and ask them to transfer that amount to you . Obviously you can't include your mommy in this. I do that all the time with my sister when it's a family trip. I spend first and equally divide the amount that we spend for common stuff like food, travel etc . Shopping is her expense separately. And we don't include parents in that sharing . (however this doesn't work with my husband's siblings though) . You can still try telling her this. .