It all depends on your individual circumstances, and tax credits won't tell you how much you are entitled to until you are in the position that you need it.
I would think your best option, assuming you don't want your mum to have both babies all week, would be to investigate the possibility of a childminder / nursery for a couple of days per week to give your mum a rest. Or investigating the possibility of going part time? Your benefit entitlement is totally dependant on your income, your partner's income, how many hours you work, howmany your partner works, what you pay for childcare, etc. You could also look into whether your workplace operates a childcare voucher scheme?