Sharing breast milk

This may be a little odd of a post to some people. But I guess I'm just looking to see if others have done this or if this is okay to do.

My sister just had a baby 2-3 days ago. She is having issues with breastfeeding. She wants to breastfeed and is breastfeeding, however the peditrician is concerned because her son is dehydrated.

She is not producing enough milk and is supplementing with formula. She has expressed that she does not want to do formula at all. She's super upset right now.

She is pumping, though I don't think she's pumping enough because from what she is telling me, she has has been too busy since she got home from the hospital. And she says when she does pump, she's not even pumping an ounce.

Her son also lost weight so that's another thing that she is upset about. Besides that she has a lot of other things on her plate.

Now I myself am breastfeeding my own son. Though we are on the weening off process. My son just turned 2. He 2 and 2 months to be exact.

I still have breast milk. It's just mature breast milk.

I asked if she would want me to give her son some of my breast milk. And she was super ecstatic about that. So she's on board with that idea. I told her that she would have to mix her breast milk in since she still has the new milk and that's very important for her baby to have.

I have never done something like this before but I'm worried about the baby. And I don't want him to get hospitalized. For context since he's been home (he got home yesterday) he has only peed once. According to her, he went the whole day today without peeing until about an hour ago.

I'm not sure if that's possible. But either way it's not good at all and I'm just extremely worried.

So that's why I offered to help.

Has anyone done something like this before? Is it okay that he gets some mature breast milk?