Dolly and Cat are right...analyze the scene carefully after reading their explanations, and you will see.

The point of the scene, I believe, would be just to give Lady an opportunity to have a happier home. Altho they were much older than her, they pretty much treated her like a friend, not like a niece. She was still young, so they still had to treat her young to an extent.

They gave her advice when she needed it, which is really something any friend would do. Even an older one. And when they saw that Jim Dear and Darling were gone for who-knows how long, they probably knew about what Aunt Sarah did to her and how she was taken out of the pound. They probably knew how upset Lady was. And since times were different then, asking someone to share your home with you was probably considered a marriage of sorts. They both cared for her deeply, but it doesn't necessarily mean they're mates xDD

Lady was kindly declining their offer, but it wasn't because she didn't care about them. I think the reason she wanted to stay was because of the well-being of the baby. After all, it was the reason she couldn't be with Tramp. Make sense?

I think they thought if Lady married one of them it would cheer her up because she would have a mate I was never quite sure of this either but that's what I assumed.