Welcome!
You may not be able to change what your family eats- but you can control what you eat
Go exercise- that's a GREAT way to lower your blood pressure
Also, don't add salt to anything you eat, if your mom cooks just eat smaller portions or fill up on as many greens as you can. Cut out caffeine- soda is terrible for weight, so cut out your soda (if you drink a lot) intake and you should notice results within a week
And above all when you go to your appointment RELAX. If you get all tense it can raise your blood pressure but if you just breathe deep and try to relax the reading should be accurate. At most if you can't relax just go lie down for five minutes (while you are waiting for the doctor) and ask them to take your BP again.
Good luck! If you can go shopping with your mom and put in a lot of healthy foods you can eat and if you have to eat a separate dinner I'd do it. A lot of times when I cook dinner I cook what I can eat then add a dish or two the family will enjoy but I won't eat.