In your first post you say you are going on hols in 2 weeks, and in your second post you say "practically 3 weeks"including the estimated 3 days since starting the season. I dont want to sound pedantic but you need to count the days precisely as a few days here or there can make a big difference

If you take 3 days ago as day 1 of the season, that makes day 1 the nineteenth of October.
Add to the three days the EXACT number of days until the holiday, and then the 11 days at your MIL and then you will know what day of her cycle she will be on when she goes to the male dogs house. As you havent given the exact number of days, I cant be very precise for you, but if it is 14 days to your hols she will be on day 17 when you go, and on day 28 when she goes to be with the male.
now a season is said to be 21 days but a bitch can conceive after this in some cases, particularly if there is some doubt on the start date. Now I do allow my boy and girl together under direct supervision from about day 25, but I observe very closely, and she wears a pair of knickers in case of accidents. I would NOT personally be happy to leave someone else in charge of supervising at such a delicate time, and would recommend that either you get your friend to agree to total separation of dog and bitch OR make alternative arrangements.
If it is longer to your hols than the 14 days, then you may be more confident.
That is why you need to be date specific.