Here's another thought. What if you moved in together, and she died. Would you be prepared to keep her kids? Could you depend on their biological father to step up? You're a responsible man. You probably have life insurance to protect your kids, if something happened to you. Does she have life insurance? If she became incapacitated and couldn't work, would you be able to afford the mortgage on the new, big house?

Blended families face the same risks that any family faces. One partner can die, or become disabled. One of the children could become sick with a catastrophic illness. Would it be fair to your kids, if one of her kids suddenly needed a great deal of attention. It can be hard enough to love your own kids, as they become teens with bigger problems. Putting up with someone else's kids can be near impossible. You have a bigger income than she has. Would you subsidize the expense of raising her kids, so that all of the children were treated equally? Would that be fair to your children? Then there is the matter of dealing with the non-custodial parents. Will her ex-husband contribute fairly?

These points are all moot now that you've rejected her proposal. I only mention these things to reinforce how thorny a situation it could become and why you do well not to get sucked in.