Do I adopt my 7yr old step grandson?

I married an older guy and after 17 yrs no regrets. Last April my eldest stepson was diagnosed with terminal cancer. He has 4 children who we all adore. George is 7 he's lively and fun but reacted really badly to what's going on at home . His mum can't cope when his Dad's having chemotherapy and neither can he. It seems the relationship between them has broken down completely. George has been staying with us till dad feels well enough to cope. His mum practically begged us to have him stay longer and I don't know what to say. I feel it's wrong to do p-the-parcel with him. I know my husband would love to have George on a full time basis but I've never been a mum and I don't have a clue what to do.