My observation has been that sometimes even the most respectful children can act in way that are out of character. They can question and challenge. They will claim that they are just acting out of concern that their parent should not make a bad decision that hasn’t been thought through, etc. Though this concern is partly the genuine motivation, there’s also sometimes a resistance to seeing a parent in a relationship with someone who isn’t Mommy…
With regard to your question about families “blending”- my limited observation is that this less of an issue when seniors marry – the children aren’t really operating in a setting in which they are expected to blend into a single unit (as happens when younger people remarry). It seems to be a bit easier for everyone to be respectful and loving. Expectations and frustration, as always, should be somewhat managed by trying to anticipate areas of conflict and using open and clear communication ….
May Hashem guide you with Siyata dishmaya and chessed