So background: I (32f) have a brother, "Dave" (35m), who's married to "Sarah" (29f). They don't have children yet. I have a son who just turned four and a three-month-old daughter with my husband (39m). My husband and I live in Belgium most of the time, but we travel back to visit my family about once a month (in England).
At home, we speak both English and French to our children (my husband is Belgian), and right now, my son is in this very sweet phase where he'll sometimes mix up the two languages and say a couple of words in English in a French sentence or vice-versa.
This has never posed a problem to us, and even the staff at his nursery have reassured us that it's very common and they tend to grow out of it once they start at school. My sister-in-law has decided that this is a problem, so when we're visiting my parents and she notices my son doing this, she'll correct him, but she does so really rudely, whereas my husband and I will just gently correct him.