Multiple mortgage lending question

[Bruce Ng]

Hi all,

Working out some family stuff on a 4 unit apartment. 
2 of 3 current owners just want to cash out.
The 3rd wants to hold onto it. 

My sister and I are exploring ways to get in with one of the owners that still wants to hold the property. 

4 units
All occupied 
Trying to keep the existing mortgage which is just $200k at 3.5%
We are working through numbers right now and haven't made an offer, but the current owners floated $1.6 mil for the purchase price. 

The mortgage broker I used for my last purchase said we won't be able to open a new loan in second place to the existing one with how much the loan amount is going to come out to.

Does anyone have any suggestions or ideas on how best to handle this while keeping the existing mortgage in place? 
or do we really need to refi the whole thing? It would be nice to bank the extra $500/mo difference.

Thanks all!