You'll probably need to check with the OU specifically - such regulations are normally specific to individual universities, and the OU has particularly unusual/unique regulations in how their degrees are classed. In general at most unis you couldn't just e.g. take an extra module to replace the lower grade one, but this may be permissible at the OU...equally the way they class degrees may not account for certain modules so this might not affect anything anyway (I vaguely recall looking at the how it's done and it was not a simple average of your results for the OU classification system, as I recall).
I'd suggest also talking to your tutor about what potential extenuating/mitigating circumstances can be applied for after the fact to account for your family situation. Usually for any short term, significant and unpredictable disruptions there are processes in place so students don't get disadvantaged by these. There is often a window in which you have to apply for them so I would suggest ringing your tutor/the OU about that tomorrow.