Worried about holiday and other junk..

Hi guys,
I am new here and for the last few weeks have been enforcing new food rules for all of us. We were big time junk food junkies at one point though I wouldn't say I have been one for quite some time now. My personal diet consists of mainly fruits, veggies, and fish.
We have three kids, a 5 year old son, a 4 year old daughter, and a 4 month old daughter who has a wheat/gluten allergy and dairy sensitivities.

I am having a hard time adjusting to no wheat and little to no dairy.. Apprently I was pretty addicted to bread and I never knew it!

Anyway, I'm trying to get us to go all organics and health foods with very little to no junk but I'm having a hard time trying to figure out how to handle holidays, birthday parties, and other junk infested gatherings..
I don't want to seem like a big meany to my other kids who up until about two months ago were allowed to eat all the good junky stuff. My son doesn't understand why he cannot have two helpings of birthday cake and ice cream at a birthday party now and is getting mixed signals since my MIL has no problem watching my older kids for a day and feeding them nothing but cookies.
I wouldn't have a problem with them having a slice of homemade birthday cake but most of the people in the family are into highly prosessed store bought stuff that makes me cringe.. I'm also starting to worry about how to handle Halloween (long way off but it's on my mind) with all that candy around how do I tell them that they're not allowed to have it all in a gentle and loving way?

My other concern is our new baby girl who like I mentioned above has wheat/gluten and dairy allergies.. How will I make sure that my little girl gets none of this stuff when her sister and brother can eat wheat and drink milk with no problem? Am I going to have to cut it out of everyone's diets? How on earth is she going to have a 1rst birthday party when she can't eat cake of ice cream? I'm really confused and have never dealt with food allergies before, though I was lactose intolerant when I was a child, I seemed to outgrow it around the time when I concieved my first baby..

I know I'm rambling I'm just kind of lost as to how we're supposed to participate in all the family food gatherings while putting some serious boundaries on the things I allow them to eat since everyone else eats mainly all junk too.

The other obstancle I'm finding I have is explaining to family members that we aren't letting the kids eat certain things. They already think I'm a right crack pot for my parenting style, my obsession with tie dye, and my nude art LOL I can see the eyes rolling already..

Any suggestions please? My husbands family Easter dinner is coming up on Sunday and I want to go armed with things to tell both the kids and the adults about why they can't eat candy all day long and why we don't eat these things anymore (My son keeps saying things like "But we used to eat that stuff and we were just fine, why can't we eat it now?")..

Thanks.
Blessed Be,