Oh those grandparents, gotta love them huh? I would feed her, wait the 3-4 hours and then feed again, if she cries/fusses, it should only last a day or two until you get back on schedule. You need your sanity! Plus I would make sure to call my mother and casually "complain" about what is happening...

Sorry....I suspect she is going through a growth spurt and needs the food.
Never let a baby cry because you don't feel like feeding her just then. She will get herself back on a reasonable schedule soon enough. During the day, keep her in the same room as you are.....so she knows it is day time and not night.....and at night but her in her room in the dark with the door shut for quiet....and she will soon sleep when she should and eat when she should.
Meanwhile....if she's hungry, feed her!
I would guess this is your first child....and I would also guess you have not had her back to the Dr. for a while.
She has a lot of growing to do in these next few months.....she won't over eat at 7 weeks.
Was she small at birth? What has she gained so far? It should be almost 2 pounds since birth.
Some babies just don't eat well in the first weeks and will be content to sleep unless you wake them. She should be eating about 20 oz a day.....depending if she is a big baby or a tiny one.
Have you changed the formula? Are the nipples getting worn? Does she have thrush....a sore mouth?.
Grand parents feeding a baby when she cries can't ruin her schedule.....there is sometning else going on here.
Linda C