Babysitting questions

So I have a 9 month old son who has been the best baby. Super calm and content little guy. I’m a stay at home mom and some friends of our asked if I’d be willing to babysit their little girl once she went back from maternity leave. My little dude has no problems at all and is really good for the most part about her being around the house. She on the other hand, her mom breastfeeds her, which is great. The bad thing is, her mom told me anytime she cries, she just throws her on the boob to calm her. I obviously can’t do that here, so this girl cries all of the time. She will let me just hold the bottle in her mouth for comfort and sleep, but the second I take it out she screams. She won’t take the pacifier either. I have to take care of my son too, so it’s definitely becoming a problem. Also my son is a great napper. He does every nap in his crib and they’re usually 2 hour naps. Ever since she’s been here, her cry wakes him up and he’s only been napping 20-30 minutes. Any advice on what to do? I’m at a loss and my son is suffering from it. I feel like he’s getting no attention or sleep. I’m trying really hard to make this work, but I don’t know what else to do. 😔

Any advice is welcome! Thanks guys!