It does say im a receptionist and what my contracted hours but thats it

Hi I currently job share 4 hrs a day 5 days a week, my working partner is going to be off for a few month due to a knee operation. Problem is if im asked there is no way I can work full time, I have 3 kids and she is likely to still be off through the summer holidays, I have looked at my contract it doesn't even state I am job share or that I have to cover sick or holidays! Only that they can vary my hours not that they can make me work more and with reasonable notice from time to time.....so can they make me work full time to cover, as any time she had called in ill they have never asked me to cover which to me implies I dont have to cover, just want to know as its not in my contract can they force anything? Thanks