I've taken on more than I can handle

Okay, so...I'm nursing my 9 month old and doing baby led weaning...he doesn't eat much food so I'm still nursing him a lot. I'm also giving my cousin 30 oz of milk a day for her baby and have been for 5 months now. Her baby was born premature and she nearly died giving birth and she ended up having serious issues with supply and asked me to help. So for the last 5 months I've been nursing my own son and also pumping 30 oz per day for her baby. I'm starting to get really worn down and overwhelmed. I don't know what to do though because I really care about my cousin and her baby and want what's best for the baby. Should I keep giving her milk? Or should I tell her I can't anymore? How would I even go about telling her this? I hate the thought of her baby having to go on formula...I did say I'd do this after all.