Frankly you seem determined to find stumbling blocks here. “They’ve asked me not to use my native language in front of them and I’m devastated.” So do it while they’re not there.
“Oh, but they’re there so often, and now I can’t teach my children my native language and it will ruin my legacy”. So everyone suggests there’s absolutely no reason why you can’t teach them; just get them to use English while the in-laws are there. “But I can’t guarantee they won’t use my native language though!!”
So welcome to real life, where children don’t always behave exactly how we want all the time. All you have to do is gently remind them.
“But the in-laws are always there, it won’t be worth teaching them; they’re in daycare all day, when will I get the chance to do it?!” Well your children were in daycare regardless of your in-laws views on your native language. You must have had some plan around this.
Stop thinking you HAVE to do everything your in-laws ask. Yes, you and your mother shouldn’t be having private conversations in front of them in a language they can’t understand, but that’s easily solved. They don’t need to all be together all the time anyway. And it doesn’t mean you can’t teach your children your native language.
Start looking for solutions instead of problems. Why not say “I’m going to give the children language lessons on Wednesdays from now on, so perhaps we could see you on Thursdays instead?”? And no “But they won’t like that, though!” or any of that mawkish moaning. If they don’t like it, tough! Learn when to use the word “No” - in both languages.