Okay, now I see that it is the OP's side of the family... not her husband's.
OP, it is your extended family. It is your children who are also affected by this.
I say it is YOUR call...
Do you want to go???
You ask if you are being unreasonable... However, I still do not know for sure what position that you are taking, so that can make that judgement call.
IMHO.. either way... whether you go, or decide not to go... Either decision is your personal decision, and either would be reasonable.
As I said, personally, for me... under no circumstances does anybody dictate what or how my kid(s) eat...
The other posters are talking about everyone being adults...
To that, I have to say... yeah right!!!!
Something tells me that the OP is not foreseeing a reasonable adult conversation.
Personally, I would tell them that as long as I am paying my portion of the rental, and feeding my own children, then I simply can not agree to such restrictions.
IMHO, the person who has the requirements/restrictions etc.. is the one responsible for meeting their own needs... It is NOT appropriate to ask others to meet 'your' wishes...
I would totally and fully RESPECT this familiys desire to observe these religious observances... (Give them their own separate kitchen cabinets, do not expect them to eat things that they do not believe that they should eat, etc....)
But it is never okay to force ones own religious practices on others...
If this SIL thinks that a full and thorough observation of LENT is that important, then she should either stay at home in her own home, or rent her own beach house or condo...
Really, expecting anyone else to follow these kinds of restrictions is not right... not right at all.