I am going to be allowed to roll over 401k starting April 1 and begin max contribution. 

Then evaluate your financial position and what you can contribute to the purchase.

You will have a better chance of getting financing, distribute the inherent risk of real estate investing, and have multiple incomes to pay rent (lowering your contribution towards rent).However, you would be wise to have a in-depth discussion about this with any partner you take on.

Therefore - your liability should be limited to the amount that you contribute to the partnership/LLC.Whether you feel 50% is fair or if you should be entitled to more depends on what each person is bringing to the table and what his/her level of experience is.The more experience a person has the more leverage he/she has to demand a higher percentage of the profits.A general contractor/property manager with 20+years of experience likely has more leverage than a contractor who is just starting out.It also depends on what you require as a rate of return.

Financially it is not a stretch for my sis and broinlaw without having my parents contribute. 

If the allocation of income/loss is not based on the capital contribution percentages, it is best to get a professional and draft a  partnership agreement.

What would you do if you wanted to pay your parents or contribute to their retirement? 

Thanks to all of you for providing and contributing to such a wealth of knowledge and, well, wealth.

If your goal is to maximize your retirement plan contributions with a SEP, then keeping things as a basic LLC might be better.If you're really concerned about keeping your brokerage and your personal commissions separate, then two separate entities, possibly either 2 S-Corps or a nested S-Corp/LLC structure might be best.So when tax season is over, sit down with your CPA and outline your goals and your issues and see what they recommend.Now if what you've got is a data entry/"tax preparer" who simply enters your data into tax forms, consider switching providers. 

specifically one that offer great one-on-one counseling, classes, and grants that contribute to closing cost/down payment!