Can my 70y mom take 50 mg of sertraline instead of afternoon at bed time with another 50mg of setraline&10mg nortiptylne?she always forget.

No, tell her Dr.: It's never a good idea to modify anything in a prescription w/out talking to the provider. They need to make the appropriate adjustment and have a record of it. This is the only way they can be responsible for her care. Hopefully either a different drug or a way to avoid the memory problem will be easy to find. Perhaps one dose will be enough, but it's the Dr's job - not yours or ours. Make a call