I can't believe any family would think its ok for their mother/mil to be looking after 4-6 kids five days week!!! Especially when she had voiced that it is difficult for her. Frankly it would be extremely difficult for most people and as a whole family I think it's sad that nobody is considering your mil's wellbeing.
I think you and your sil should find a paid babysitter to watch all 6 kids for 2 days a week and split the cost. That way you and sil get childcare at a reduce rate to a holiday care program and mil gets a few days to rest and enjoy her own life. The kids also get to spend time with their cousins. Everyone wins.