P.,
Of course you have the right to regulate what your child eats. Unfortunately, you can only truly do this when YOU are the sole person in charge of her meals. So the best you can do is feed her as healthy as you see fit while she is under your care.
When you leave her in the care of her dad, grandparents, friends, etc., she will NOT be eating what you would like for her to eat. That is the sad reality. Because no two people are exactly alike regarding what a child can or should be eating. Even health nuts will disagree about what constitutes healthy.
Trying to explain to others about why you want her to only eat healthy foods will often alienate you from them. You see, they believe they are giving them what is best, be it candy, vegetables or whatever.
It does seem a bit ridiculous that anyone would give a child under one years old candy. But people do it, because they think that's part of being a child. They believe that it brings a bit of extra sunshine into their lives and a smile and enjoy watching that. Try not to be too opinionated with people like this, because trust me, it will not help, only make things worse. Grin and bear it if you will. She will be back in your care and you can get her back on track.
I do empathize with you. I have had struggles of my own regarding this. I had a babysitter when my girls were very small who came to our house to babysit. I kept healthy foods, with absolutely no junk at our house when she came over. When I went to college for a semester to finally get my degree I used this babysitter as a full-time nanny during that time. Because I received a scholarship, I figured that the money I would have spent on tuition would pay for her services. I started to notice my older daughter getting pudgier. It made no sense, because I went out of my way to prepare healthy meals ahead of time for when I was gone. That food would always be gone when I came back. But the cat got out of the bag when I found a stash of junk in a pillowcase/pillow that the babysitter accidentally left one day. I asked my daughter, who was about 5 at the time about it and she told me the babysitter gave her treats every day. I was not happy and confronted her. She felt very guilty, I could tell. But the damage had already been done.
I agree with the posters who said that being too opinionated about your food choices can come across as judgmental and arrogant. Be careful with the in-laws. I, too, learned the hard way with this.
I thought I was being respectful with my MIL and told her that our pediatrician recommended the food we were giving our daughters. She ignored anything regarding our preferences and gave them (and still does) popsicles and ice-cream right before bedtime, and bombarded them with cookies and cakes all day long. Then she would be the first one to comment on my older daughter's weight. I tried an approach of "What do you think would be healthy food for the girls?" in a very nice way. She told me that she never let her kids eat junk ever, but that she was their grandmother and it was her right to let them eat whatever she wanted them to while they are at her house. She set me straight, huh!
Another thing (I know I'm going on and on about this!): no matter how healthy of food you give your daughter at home, when she gets to school one day and sees the cool food that some of the other kids get to eat, she will protest and rebel. Like me, you will stick to your guns. But know that when she is having a playdate, or even from the teachers, she is going to get the junk sometimes.
Like others have said, you just have to relax and let it go. My older daughter struggles with her weight - she has always looked pudgy from the time she was born, in spite of being active and trying to keep her diet junk-free. She is finally beginning to slim out, because of all the activities she is involved in (gymnastics, tennis, cycling, taking walks, etc.). She still gets junk from well-meaning friends and relatives and often resists my healthy menu, but it has gotten better.
P.S. You are on the right track with your home-cookes meals. Kudos to you! Some of the Gerber food in the jars try to imitate apple pie and such, but they just can't compare to the home-made mashed-up version :)
Many blessings to you