We are dual income household and save a lot of our money. We bought our current condo in JP 2 years ago - don't want to sell yet in this Boston RE market. We have about $150k cash saved up in a high return savings account (post emergency fund), which is enough for a DP on a place in a better location for our needs, still in the city, possibly South Boston. We’re aware that our living space will not get bigger if we move into this new primary residence. We were reluctant to rent our place out before due to the hassles, but suddenly have the option of renting our current place to trustworthy relatives. Their rent would cover our mortgage payment on the condo. Before this came up, we were wracking our brains with the best way to invest this money. Issues are:

We would be looking to move to a place with better schools (suburbs) in ~5 years when our kids start going to school, so our initial plan was just to keep saving until then and put more down on a place in the suburbs, and rent out our current place then. If we do the buy/rent combo we're debating now, we'd probably sell one of the places when we move out to the burbs, and keep the other one as rental property. We're not sure what else is better to do with the money if not RE. We do have a chunk in an investment fund, but reluctant to put the rest of it in due to potentially wanting to use it to do RE investing. 

Thanks in advance for any advice.