Before reading the response to your question, have a look at the numerology meaning of the number 11. It's a wonderful number.

Generally, a person will see (and remember seeing) certain numbers repetitively when the number has personal meaning or emotional significance.

Your own birth month being the number 11 would predispose you to seeing that number. My thought is that you may have been the first to see the number repeatedly. After talking about it, the rest of the family also started seeing the number.

The member of your family with a 05/21 birthday (permission to publish name wasn't provided) has a destiny number 11. And the member with a 05/08 birthday has a personality number 11. Those may have sufficient influence for the number 11 to be noticed more often than would otherwise be the case.

I see no 11s in your grandmother's numerology chart for a major number.

Because your entire family is involved, I'm thinking that talking about it and wondering about it predisposes the individuals to notice the 11 when it's on the clock. More than just noticing (because other times are also noticed), seeing the number 11 is definitely remembered and other times are less likely to be remembered.

See Why People See Certain Numbers All the Time for an explanation of how that works.

When many numbers are seen during the day, as is the case for most people, but only certain ones are remembered, it seems that the remembered numbers are being seen more often than other numbers. Which might be the fact, or might not; without a physical record to refer to, it can't be established with certainty. But it does seem so to the person doing the remembering.